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/Frontend/Frontend.php

@@ -0,0 +1,84 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Frontend;
+
+use Flarum\Api\Client;
+use Flarum\Api\Controller\ShowForumController;
+use Illuminate\Contracts\View\Factory;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class Frontend
+{
+    /**
+     * @var Factory
+     */
+    protected $view;
+
+    /**
+     * @var Client
+     */
+    protected $api;
+
+    /**
+     * @var callable[]
+     */
+    protected $content = [];
+
+    public function __construct(Factory $view, Client $api)
+    {
+        $this->view = $view;
+        $this->api = $api;
+    }
+
+    /**
+     * @param callable $content
+     */
+    public function content(callable $content)
+    {
+        $this->content[] = $content;
+    }
+
+    public function document(Request $request): Document
+    {
+        $forumDocument = $this->getForumDocument($request);
+
+        $document = new Document($this->view, $forumDocument);
+
+        $this->populate($document, $request);
+
+        return $document;
+    }
+
+    protected function populate(Document $document, Request $request)
+    {
+        foreach ($this->content as $content) {
+            $content($document, $request);
+        }
+    }
+
+    private function getForumDocument(Request $request): array
+    {
+        $actor = $request->getAttribute('actor');
+
+        $this->api->setErrorHandler(null);
+
+        return $this->getResponseBody(
+            $this->api->send(ShowForumController::class, $actor)
+        );
+    }
+
+    private function getResponseBody(Response $response): array
+    {
+        return json_decode($response->getBody(), true);
+    }
+}