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/Forum/ForumServiceProvider.php

@@ -13,7 +13,6 @@ use Flarum\Extension\Event\Disabled;
 use Flarum\Extension\Event\Enabled;
 use Flarum\Formatter\Formatter;
 use Flarum\Foundation\AbstractServiceProvider;
-use Flarum\Foundation\Application;
 use Flarum\Foundation\ErrorHandling\Registry;
 use Flarum\Foundation\ErrorHandling\Reporter;
 use Flarum\Foundation\ErrorHandling\ViewFormatter;
@@ -70,14 +69,14 @@ class ForumServiceProvider extends AbstractServiceProvider
             ];
         });
 
-        $this->app->singleton('flarum.forum.handler', function (Application $app) {
+        $this->app->singleton('flarum.forum.handler', function () {
             $pipe = new MiddlewarePipe;
 
             // All requests should first be piped through our global error handler
             $pipe->pipe(new HttpMiddleware\HandleErrors(
-                $app->make(Registry::class),
-                $app->inDebugMode() ? $app->make(WhoopsFormatter::class) : $app->make(ViewFormatter::class),
-                $app->tagged(Reporter::class)
+                $this->app->make(Registry::class),
+                $this->app['flarum']->inDebugMode() ? $this->app->make(WhoopsFormatter::class) : $this->app->make(ViewFormatter::class),
+                $this->app->tagged(Reporter::class)
             ));
 
             foreach ($this->app->make('flarum.forum.middleware') as $middleware) {