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/Extension/ExtensionServiceProvider.php

@@ -11,7 +11,6 @@ namespace Flarum\Extension;
 
 use Flarum\Extension\Event\Disabling;
 use Flarum\Foundation\AbstractServiceProvider;
-use Illuminate\Contracts\Container\Container;
 
 class ExtensionServiceProvider extends AbstractServiceProvider
 {
@@ -27,8 +26,8 @@ class ExtensionServiceProvider extends AbstractServiceProvider
         // listener on the app rather than in the service provider's boot method
         // below, so that extensions have a chance to register things on the
         // container before the core boots up (and starts resolving services).
-        $this->app->booting(function (Container $app) {
-            $app->make('flarum.extensions')->extend($app);
+        $this->app['flarum']->booting(function () {
+            $this->app->make('flarum.extensions')->extend($this->app);
         });
     }