Use isomorphic-fetch instead of node-fetch

Summary:
Paired with @dandelionmane.

Test Plan:
```
$ CI=true yarn test
$ yarn backend
$ GITHUB_TOKEN="<your_token>" src/plugins/github/fetchGitHubRepoTest.sh
```

wchargin-branch: isomorphic-fetch

config/webpack.config.backend.js

@@ -75,9 +75,5 @@ module.exports = {
         process.env.NODE_ENV || "development"
       ),
     }),
-    // See:
-    //   - https://github.com/andris9/encoding/issues/16 (the culprit)
-    //   - https://github.com/bitinn/node-fetch/issues/41 (the solution)
-    new webpack.IgnorePlugin(/\/iconv-loader$/),
   ],
 };

package.json

@@ -29,11 +29,11 @@
     "fs-extra": "3.0.1",
     "html-webpack-plugin": "2.29.0",
     "husky": "^0.14.3",
+    "isomorphic-fetch": "^2.2.1",
     "jest": "20.0.4",
     "json-stable-stringify": "^1.0.1",
     "lint-staged": "^6.1.1",
     "lodash.isequal": "^4.5.0",
-    "node-fetch": "^2.1.1",
     "object-assign": "4.1.1",
     "postcss-flexbugs-fixes": "3.2.0",
     "postcss-loader": "2.0.8",

src/plugins/github/fetchGitHubRepo.js

@@ -4,7 +4,7 @@
  * docstring of the default export for more details.
  */
 
-import fetch from "node-fetch";
+import fetch from "isomorphic-fetch";
 
 /**
  * Scrape data from a GitHub repo using the GitHub API.

yarn.lock

@@ -3551,7 +3551,7 @@ isobject@^2.0.0:
   dependencies:
     isarray "1.0.0"
 
-isomorphic-fetch@^2.1.1:
+isomorphic-fetch@^2.1.1, isomorphic-fetch@^2.2.1:
   version "2.2.1"
   resolved "https://registry.yarnpkg.com/isomorphic-fetch/-/isomorphic-fetch-2.2.1.tgz#611ae1acf14f5e81f729507472819fe9733558a9"
   dependencies:
@@ -4466,10 +4466,6 @@ node-fetch@^1.0.1:
     encoding "^0.1.11"
     is-stream "^1.0.1"
 
-node-fetch@^2.1.1:
-  version "2.1.1"
-  resolved "https://registry.yarnpkg.com/node-fetch/-/node-fetch-2.1.1.tgz#369ca70b82f50c86496104a6c776d274f4e4a2d4"
-
 node-forge@0.7.1:
   version "0.7.1"
   resolved "https://registry.yarnpkg.com/node-forge/-/node-forge-0.7.1.tgz#9da611ea08982f4b94206b3beb4cc9665f20c300"