Frontend refactor (#1471)

Refactor Frontend + Asset code

- Use Laravel's Filesystem component for asset IO, meaning theoretically
  assets should be storable on S3 etc.

- More reliable checking for asset recompilation when debug mode is on,
  so you don't have to constantly delete the compiled assets to force
  a recompile. Should also fix issues with locale JS files being
  recompiled with the same name and cached.

- Remove JavaScript minification, because it will be done by Webpack
  (exception is for the TextFormatter JS).

- Add support for JS sourcemaps.

- Separate frontend view and assets completely. This is an important
  distinction because frontend assets are compiled independent of a
  request, whereas putting together a view depends on a request.

- Bind frontend view/asset factory instances to the container (in
  service providers) rather than subclassing. Asset and content
  populators can be added to these factories – these are simply objects
  that populate the asset compilers or the view with information.

- Add RouteHandlerFactory functions that make it easy to hook up a
  frontend controller with a frontend instance ± some content.

- Remove the need for "nojs"

- Fix cache:clear command

- Recompile assets when settings/enabled extensions change

src/Admin/Content/AdminPayload.php

@@ -0,0 +1,60 @@
+<?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\Admin\Content;
+
+use Flarum\Extension\ExtensionManager;
+use Flarum\Frontend\Content\ContentInterface;
+use Flarum\Frontend\HtmlDocument;
+use Flarum\Group\Permission;
+use Flarum\Settings\SettingsRepositoryInterface;
+use Illuminate\Database\ConnectionInterface;
+use Psr\Http\Message\ServerRequestInterface as Request;
+
+class AdminPayload implements ContentInterface
+{
+    /**
+     * @var SettingsRepositoryInterface
+     */
+    protected $settings;
+
+    /**
+     * @var ExtensionManager
+     */
+    protected $extensions;
+
+    /**
+     * @var ConnectionInterface
+     */
+    protected $db;
+
+    /**
+     * @param SettingsRepositoryInterface $settings
+     * @param ExtensionManager $extensions
+     * @param ConnectionInterface $db
+     */
+    public function __construct(SettingsRepositoryInterface $settings, ExtensionManager $extensions, ConnectionInterface $db)
+    {
+        $this->settings = $settings;
+        $this->extensions = $extensions;
+        $this->db = $db;
+    }
+
+    public function populate(HtmlDocument $document, Request $request)
+    {
+        $document->payload['settings'] = $this->settings->all();
+        $document->payload['permissions'] = Permission::map();
+        $document->payload['extensions'] = $this->extensions->getExtensions()->toArray();
+
+        $document->payload['phpVersion'] = PHP_VERSION;
+        $document->payload['mysqlVersion'] = $this->db->selectOne('select version() as version')->version;
+    }
+}