Split up Application and Container

- Stop trying to implement Laravel's Application contract, which
  has no value for us.
- Stop inheriting from the Container, injecting one works equally
  well and does not clutter up the interfaces.
- Inject the Paths collection instead of unwrapping it again, for
  better encapsulation.

This brings us one step closer toward upgrading our Laravel
components (#2055), because we no longer need to adopt the changes
to the Application contract.

src/Frontend/FrontendServiceProvider.php

@@ -24,11 +24,11 @@ class FrontendServiceProvider extends AbstractServiceProvider
                 $assets = new Assets(
                     $name,
                     $this->app->make('filesystem')->disk('flarum-assets'),
-                    $this->app->storagePath()
+                    $this->app['flarum']->storagePath()
                 );
 
                 $assets->setLessImportDirs([
-                    $this->app->vendorPath().'/components/font-awesome/less' => ''
+                    $this->app['flarum']->vendorPath().'/components/font-awesome/less' => ''
                 ]);
 
                 $assets->css([$this, 'addBaseCss']);