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

src/Admin/Content/AdminPayload.php

@@ -12,8 +12,7 @@
 namespace Flarum\Admin\Content;
 
 use Flarum\Extension\ExtensionManager;
-use Flarum\Frontend\Content\ContentInterface;
-use Flarum\Frontend\HtmlDocument;
+use Flarum\Frontend\Document;
 use Flarum\Group\Permission;
 use Flarum\Settings\Event\Deserializing;
 use Flarum\Settings\SettingsRepositoryInterface;
@@ -21,7 +20,7 @@ use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Database\ConnectionInterface;
 use Psr\Http\Message\ServerRequestInterface as Request;
 
-class AdminPayload implements ContentInterface
+class AdminPayload
 {
     /**
      * @var SettingsRepositoryInterface
@@ -52,7 +51,7 @@ class AdminPayload implements ContentInterface
         $this->events = $events;
     }
 
-    public function __invoke(HtmlDocument $document, Request $request)
+    public function __invoke(Document $document, Request $request)
     {
         $settings = $this->settings->all();