Refactor frontend code to allow for extension of assets

- Simpler class naming:
    Frontend\CompilerFactory → Frontend\Assets
    Frontend\HtmlDocumentFactory → Frontend\Frontend
    Frontend\HtmlDocument → Frontend\Document

- Remove AssetInterface and simply collect callbacks in Frontend\Assets
  instead

- Remove ContentInterface because it serves no purpose (never type-
  hinted or type-checked)

- Commit and add asset URLs to the Document via a content callback
  instead of in the Document factory class itself

- Add translations and locale assets to Assets separate to the assets
  factory, as non-forum/admin asset bundles probably won't want them

- Update Frontend Extender to allow the creation of new asset bundles

- Make custom LESS validation listener a standalone class instead of
  extending RecompileFrontendAssets

framework/core/src/Http/RouteHandlerFactory.php

@@ -46,7 +46,7 @@ class RouteHandlerFactory
     public function toFrontend(string $frontend, string $content = null)
     {
         return $this->toController(function (Container $container) use ($frontend, $content) {
-            $frontend = $container->make("flarum.$frontend.frontend");
+            $frontend = $container->make("flarum.frontend.$frontend");
 
             if ($content) {
                 $frontend->add($container->make($content));