Rename `makeWebpackConfig` to `webpack.config.web` (#770)

Summary:
The distinction was useful while `makeWebpackConfig` was being developed
(between #562 and #570), but is now confusing: we have a web config and
a backend config, and it is clearer if we name them as such.

Test Plan:
All of `yarn start`, `yarn build`, and `yarn test --full` work.

wchargin-branch: webpack-config-web

package.json

@@ -71,8 +71,8 @@
   "scripts": {
     "prettify": "prettier --write '**/*.js'",
     "check-pretty": "prettier --list-different '**/*.js'",
-    "start": "NODE_ENV=development webpack-dev-server --config config/makeWebpackConfig.js",
-    "build": "NODE_ENV=production webpack --config config/makeWebpackConfig.js",
+    "start": "NODE_ENV=development webpack-dev-server --config config/webpack.config.web.js",
+    "build": "NODE_ENV=production webpack --config config/webpack.config.web.js",
     "backend": "node scripts/backend.js",
     "test": "node ./config/test.js",
     "unit": "BABEL_ENV=test NODE_ENV=test jest --env=jsdom",