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.blade.php

@@ -1,23 +1,23 @@
 <div id="flarum-loading" style="display: none">
-  {{ $translator->trans('core.views.content.loading_text') }}
+    {{ $translator->trans('core.views.content.loading_text') }}
 </div>
 
-@if ($allowJs)
-  <noscript>
+<noscript>
     <div class="Alert">
-      <div class="container">
-        {{ $translator->trans('core.views.content.javascript_disabled_message') }}
-      </div>
+        <div class="container">
+            {{ $translator->trans('core.views.content.javascript_disabled_message') }}
+        </div>
     </div>
+</noscript>
 
+<div id="flarum-loading-error" style="display: none">
+    <div class="Alert">
+        <div class="container">
+            {{ $translator->trans('core.views.content.load_error_message') }}
+        </div>
+    </div>
+</div>
+
+<noscript id="flarum-content">
     {!! $content !!}
-  </noscript>
-@else
-  <div class="Alert Alert--error">
-    <div class="container">
-      {{ $translator->trans('core.views.content.load_error_message') }}
-    </div>
-  </div>
-
-  {!! $content !!}
-@endif
+</noscript>