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/Formatter/FormatterServiceProvider.php

@@ -24,7 +24,7 @@ class FormatterServiceProvider extends AbstractServiceProvider
             return new Formatter(
                 new Repository($container->make('cache.filestore')),
                 $container->make('events'),
-                $this->app->storagePath().'/formatter'
+                $this->app['flarum']->storagePath().'/formatter'
             );
         });