Upgrade babel to 7

I moved `config/babel.js` to `.babelrc.js` because it seemed like babel
7 really wanted that. I also blew away our (complicated, copied from
create-react-app) config and replaced it with a much, much simpler one.

Test plan: `yarn test` passes, `yarn start` still serves a working
server, and `scripts/build_static_site.sh` still produces a working
site.

Possibly we lost some nice features re: React debugging; if so I'll add
them back as I miss them.

.babelrc.js

@@ -0,0 +1,22 @@
+// @flow
+
+const presets = [
+  [
+    "@babel/preset-env",
+    {
+      targets: {
+        edge: "17",
+        firefox: "60",
+        chrome: "67",
+        safari: "11.1",
+        node: true,
+      },
+    },
+  ],
+  "@babel/preset-react",
+  "@babel/preset-flow",
+];
+
+const plugins = ["@babel/plugin-proposal-class-properties"];
+
+module.exports = {presets, plugins};

config/babel.js

@@ -1,153 +0,0 @@
-// @flow
-
-/**
- * Copyright (c) 2018-present, SourceCred Authors.
- * Copyright (c) 2015-2018, Facebook, Inc.
- *
- * Forked from babel-preset-react-app/index.js, which is released under
- * the MIT license.
- */
-
-const plugins = [
-  // Necessary to include regardless of the environment because
-  // in practice some other transforms (such as object-rest-spread)
-  // don't work without it: https://github.com/babel/babel/issues/7215
-  require.resolve("babel-plugin-transform-es2015-destructuring"),
-  // class { handleClick = () => { } }
-  require.resolve("babel-plugin-transform-class-properties"),
-  // The following two plugins use Object.assign directly, instead of Babel's
-  // extends helper. Note that this assumes `Object.assign` is available.
-  // { ...todo, completed: true }
-  [
-    require.resolve("babel-plugin-transform-object-rest-spread"),
-    {
-      useBuiltIns: true,
-    },
-  ],
-  // Transforms JSX
-  [
-    require.resolve("babel-plugin-transform-react-jsx"),
-    {
-      useBuiltIns: true,
-    },
-  ],
-  // Polyfills the runtime needed for async/await and generators
-  [
-    require.resolve("babel-plugin-transform-runtime"),
-    {
-      helpers: false,
-      polyfill: false,
-      regenerator: true,
-    },
-  ],
-];
-
-// This is similar to how `env` works in Babel:
-// https://babeljs.io/docs/usage/babelrc/#env-option
-// We are not using `env` because it’s ignored in versions > babel-core@6.10.4:
-// https://github.com/babel/babel/issues/4539
-// https://github.com/facebookincubator/create-react-app/issues/720
-// It’s also nice that we can enforce `NODE_ENV` being specified.
-var env = process.env.BABEL_ENV || process.env.NODE_ENV;
-var backend = process.env.SOURCECRED_BACKEND === "true";
-if (env !== "development" && env !== "test" && env !== "production") {
-  const stringified = env === undefined ? "undefined" : JSON.stringify(env);
-  throw new Error(
-    "Using `babel-preset-react-app` requires that you specify `NODE_ENV` or " +
-      '`BABEL_ENV` environment variables. Valid values are "development", ' +
-      '"test", and "production". Instead, received: ' +
-      stringified +
-      "."
-  );
-}
-
-if (env === "development" || env === "test") {
-  // The following two plugins are currently necessary to make React warnings
-  // include more valuable information. They are included here because they are
-  // currently not enabled in babel-preset-react. See the below threads for more info:
-  // https://github.com/babel/babel/issues/4702
-  // https://github.com/babel/babel/pull/3540#issuecomment-228673661
-  // https://github.com/facebookincubator/create-react-app/issues/989
-  plugins.push.apply(plugins, [
-    // Adds component stack to warning messages
-    require.resolve("babel-plugin-transform-react-jsx-source"),
-    // Adds __self attribute to JSX which React will use for some warnings
-    require.resolve("babel-plugin-transform-react-jsx-self"),
-  ]);
-}
-
-if (env === "test") {
-  module.exports = {
-    presets: [
-      // ES features necessary for user's Node version
-      [
-        require("babel-preset-env").default,
-        {
-          targets: {
-            node: "current",
-          },
-        },
-      ],
-      // JSX, Flow
-      require.resolve("babel-preset-react"),
-    ],
-    plugins: [
-      ...plugins,
-      // Compiles import() to a deferred require()
-      require.resolve("babel-plugin-dynamic-import-node"),
-    ],
-  };
-} else {
-  module.exports = {
-    presets: [
-      // Latest stable ECMAScript features
-      [
-        require.resolve("babel-preset-env"),
-        {
-          targets: backend
-            ? {
-                node: "current",
-              }
-            : {
-                ie: 9,
-                uglify: true,
-              },
-          // Disable polyfill transforms
-          useBuiltIns: false,
-        },
-      ],
-      // JSX, Flow
-      require.resolve("babel-preset-react"),
-    ],
-    plugins: [
-      ...plugins,
-      ...(backend
-        ? [
-            // Must come before `babel-plugin-transform-regenerator`.
-            require.resolve("babel-plugin-transform-es2015-for-of"),
-          ]
-        : [
-            // function* () { yield 42; yield 43; }
-            [
-              require.resolve("babel-plugin-transform-regenerator"),
-              {
-                // Async functions are converted to generators by
-                // babel-preset-env
-                async: false,
-              },
-            ],
-          ]),
-      // Adds syntax support for import()
-      require.resolve("babel-plugin-syntax-dynamic-import"),
-    ],
-  };
-
-  if (env === "production") {
-    // Optimization: hoist JSX that never changes out of render()
-    // Disabled because of issues: https://github.com/facebookincubator/create-react-app/issues/553
-    // TODO: Enable again when these issues are resolved.
-    // plugins.push.apply(plugins, [
-    //   require.resolve('babel-plugin-transform-react-constant-elements')
-    // ]);
-  }
-}

package.json

@@ -33,13 +33,14 @@
     "whatwg-fetch": "2.0.3"
   },
   "devDependencies": {
-    "babel-core": "6.26.3",
-    "babel-eslint": "10.0.2",
-    "babel-jest": "20.0.3",
-    "babel-loader": "7.1.2",
-    "babel-plugin-transform-es2015-for-of": "^6.23.0",
-    "babel-preset-react-app": "^3.1.1",
-    "babel-runtime": "6.26.0",
+    "@babel/core": "^7.5.4",
+    "@babel/plugin-proposal-class-properties": "^7.5.0",
+    "@babel/preset-env": "^7.5.4",
+    "@babel/preset-flow": "^7.0.0",
+    "@babel/preset-react": "^7.0.0",
+    "babel-eslint": "^10.0.2",
+    "babel-jest": "^24.8.0",
+    "babel-loader": "^8.0.6",
     "copy-webpack-plugin": "^4.5.2",
     "css-loader": "0.28.7",
     "dotenv": "8.0.0",
@@ -68,9 +69,6 @@
   },
   "greenkeeper": {
     "ignore": [
-      "babel-jest",
-      "babel-loader",
-      "babel-preset-react-app",
       "copy-webpack-plugin",
       "css-loader",
       "file-loader",
@@ -134,11 +132,6 @@
       "node"
     ]
   },
-  "babel": {
-    "presets": [
-      "./config/babel"
-    ]
-  },
   "files": [
     "/bin",
     "/build"