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/Forum/Content/AssertRegistered.php

@@ -11,16 +11,15 @@
 
 namespace Flarum\Forum\Content;
 
-use Flarum\Frontend\Content\ContentInterface;
-use Flarum\Frontend\HtmlDocument;
+use Flarum\Frontend\Document;
 use Flarum\User\AssertPermissionTrait;
 use Psr\Http\Message\ServerRequestInterface as Request;
 
-class AssertRegistered implements ContentInterface
+class AssertRegistered
 {
     use AssertPermissionTrait;
 
-    public function __invoke(HtmlDocument $document, Request $request)
+    public function __invoke(Document $document, Request $request)
     {
         $this->assertRegistered($request->getAttribute('actor'));
     }