Frontend refactor (#1471)

Refactor Frontend + Asset code

- Use Laravel's Filesystem component for asset IO, meaning theoretically
  assets should be storable on S3 etc.

- More reliable checking for asset recompilation when debug mode is on,
  so you don't have to constantly delete the compiled assets to force
  a recompile. Should also fix issues with locale JS files being
  recompiled with the same name and cached.

- Remove JavaScript minification, because it will be done by Webpack
  (exception is for the TextFormatter JS).

- Add support for JS sourcemaps.

- Separate frontend view and assets completely. This is an important
  distinction because frontend assets are compiled independent of a
  request, whereas putting together a view depends on a request.

- Bind frontend view/asset factory instances to the container (in
  service providers) rather than subclassing. Asset and content
  populators can be added to these factories – these are simply objects
  that populate the asset compilers or the view with information.

- Add RouteHandlerFactory functions that make it easy to hook up a
  frontend controller with a frontend instance ± some content.

- Remove the need for "nojs"

- Fix cache:clear command

- Recompile assets when settings/enabled extensions change

views/frontend/content/discussion.blade.php

@@ -1,5 +1,5 @@
 <div class="container">
-    <h2>{{ $document->data->attributes->title }}</h2>
+    <h2>{{ $apiDocument->data->attributes->title }}</h2>
 
     <div>
         @foreach ($posts as $post)
@@ -15,11 +15,11 @@
         @endforeach
     </div>
 
-    @if (isset($document->links->prev))
+    @if (isset($apiDocument->links->prev))
         <a href="{{ $url(['page' => $page - 1]) }}">&laquo; {{ $translator->trans('core.views.discussion.previous_page_button') }}</a>
     @endif
 
-    @if (isset($document->links->next))
+    @if (isset($apiDocument->links->next))
         <a href="{{ $url(['page' => $page + 1]) }}">{{ $translator->trans('core.views.discussion.next_page_button') }} &raquo;</a>
     @endif
 </div>