Squashing asset pipeline bugs and other stuff

1. Update dependencies
   - Add font-awesome as dependency
   - Switch to Bootstrap 4 dev branch
   - Remove eslint airbnb style (we're not using it)

2. Imported images and fonts should go into dist/images
   and dist/fonts, respectively

3. Only lint our own code, not external code (fixes #1633)

.eslintrc

@@ -2,7 +2,7 @@
   "root": true,
   "extends": "eslint:recommended",
   "globals": {
-    "wp": true,
+    "wp": true
   },
   "env": {
     "browser": true,