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/AdminServiceProvider.php

@@ -14,6 +14,9 @@ namespace Flarum\Admin;
 use Flarum\Event\ConfigureMiddleware;
 use Flarum\Foundation\AbstractServiceProvider;
 use Flarum\Foundation\Application;
+use Flarum\Frontend\AddLocaleAssets;
+use Flarum\Frontend\AddTranslations;
+use Flarum\Frontend\Compiler\Source\SourceCollector;
 use Flarum\Frontend\RecompileFrontendAssets;
 use Flarum\Http\Middleware as HttpMiddleware;
 use Flarum\Http\RouteCollection;
@@ -62,17 +65,31 @@ class AdminServiceProvider extends AbstractServiceProvider
             $pipe->pipe(new HttpMiddleware\DispatchRoute($this->app->make('flarum.admin.routes')));
         });
 
-        $this->app->bind('flarum.admin.assets', function () {
-            return $this->app->make('flarum.frontend.assets.defaults')('admin');
+        $this->app->bind('flarum.assets.admin', function () {
+            /** @var \Flarum\Frontend\Assets $assets */
+            $assets = $this->app->make('flarum.assets.factory')('admin');
+
+            $assets->js(function (SourceCollector $sources) {
+                $sources->addFile(__DIR__.'/../../js/dist/admin.js');
+            });
+
+            $assets->css(function (SourceCollector $sources) {
+                $sources->addFile(__DIR__.'/../../less/admin.less');
+            });
+
+            $this->app->make(AddTranslations::class)->forFrontend('admin')->to($assets);
+            $this->app->make(AddLocaleAssets::class)->to($assets);
+
+            return $assets;
         });
 
-        $this->app->bind('flarum.admin.frontend', function () {
-            $view = $this->app->make('flarum.frontend.view.defaults')('admin');
+        $this->app->bind('flarum.frontend.admin', function () {
+            /** @var \Flarum\Frontend\Frontend $frontend */
+            $frontend = $this->app->make('flarum.frontend.factory')('admin');
 
-            $view->setAssets($this->app->make('flarum.admin.assets'));
-            $view->add($this->app->make(Content\AdminPayload::class));
+            $frontend->content($this->app->make(Content\AdminPayload::class));
 
-            return $view;
+            return $frontend;
         });
     }
 
@@ -87,7 +104,7 @@ class AdminServiceProvider extends AbstractServiceProvider
 
         $this->app->make('events')->subscribe(
             new RecompileFrontendAssets(
-                $this->app->make('flarum.admin.assets'),
+                $this->app->make('flarum.assets.admin'),
                 $this->app->make('flarum.locales')
             )
         );

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();