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.

stubs/extension/js/bootstrap.js

@@ -1,8 +0,0 @@
-import { extend, override } from 'flarum/extension-utils';
-import app from 'flarum/app';
-
-app.initializers.add('{{name}}', function() {
-
-  // @todo
-
-});

stubs/extension/js/src/main.js

@@ -0,0 +1,4 @@
+import { extend } from 'flarum/extend';
+import app from 'flarum/app';
+
+// TODO