Rework extension bootstrapping

System JS modules don't execute when they're registered, so we need to
import them explicitly. While we're at it, we may as well make the
locale bootstrapper a module too.

js/forum/Gulpfile.js

@@ -30,7 +30,6 @@ gulp({
     'src/**/*.js',
     '../lib/**/*.js'
   ],
-  bootstrapFiles: [],
   modulePrefix: 'flarum',
   externalHelpers: true,
   outputFile: 'dist/app.js'