Big front-end asset/filestructure refactor

- Extract shared Ember components into a “flarum-common” ember-cli
addon. This can be used by both the forum + admin Ember apps, keeping
things DRY
- Move LESS styles into their own top-level directory and do a similar
thing (extract common styles)
- Add LESS/JS compilation and versioning to PHP (AssetManager)
- Set up admin entry point

(Theoretical) upgrade instructions:
- Delete everything in [app_root]/public
- Set up tooling in forum/admin Ember apps (npm install/update, bower
install/update) and then build them (ember build)
- php artisan vendor:publish
- Upgrade flarum/flarum repo (slight change in a config file)
- If you need to trigger a LESS/JS recompile, delete the .css/.js files
in [app_root]/public/flarum. I set up LiveReload to do this for me when
I change files in less/ or ember/

Todo:
- Start writing admin app!
- Remove bootstrap/font-awesome from repo and instead depend on their
composer packages? Maybe? (Bower is not an option here)

src/Admin/AdminServiceProvider.php

@@ -0,0 +1,45 @@
+<?php namespace Flarum\Admin;
+
+use Illuminate\Support\ServiceProvider;
+use Flarum\Web\AssetManager;
+
+class AdminServiceProvider extends ServiceProvider
+{
+    /**
+     * Bootstrap the application events.
+     *
+     * @return void
+     */
+    public function boot()
+    {
+        $root = __DIR__.'/../..';
+
+        $this->loadViewsFrom($root.'/views', 'flarum.admin');
+
+        $assetManager = $this->app['flarum.admin.assetManager'];
+
+        $assetManager->addFile([
+            $root.'/ember/admin/dist/assets/vendor.js',
+            $root.'/ember/admin/dist/assets/flarum-admin.js',
+            $root.'/less/admin/app.less'
+        ]);
+
+        $this->publishes([
+            $root.'/public/fonts' => public_path('flarum/fonts')
+        ]);
+
+        include __DIR__.'/routes.php';
+    }
+
+    /**
+     * Register the service provider.
+     *
+     * @return void
+     */
+    public function register()
+    {
+        $this->app['flarum.admin.assetManager'] = $this->app->share(function ($app) {
+            return new AssetManager($app['files'], $app['path.public'].'/flarum', 'admin');
+        });
+    }
+}