Get rid of Server classes for Admin, API and Forum

The various middleware can be registered in the service provider,
and the rest of the logic can all go through one single front
controller (index.php in flarum/flarum, and Flarum\Http\Server in
flarum/core).

This will also simplify the necessary server setup, as only one
rewrite rule remains.

src/Api/ApiServiceProvider.php

@@ -12,17 +12,28 @@
 namespace Flarum\Api;
 
 use Flarum\Api\Controller\AbstractSerializeController;
+use Flarum\Api\Middleware\FakeHttpMethods;
+use Flarum\Api\Middleware\HandleErrors;
 use Flarum\Api\Serializer\AbstractSerializer;
 use Flarum\Api\Serializer\NotificationSerializer;
 use Flarum\Event\ConfigureApiRoutes;
+use Flarum\Event\ConfigureMiddleware;
 use Flarum\Event\ConfigureNotificationTypes;
 use Flarum\Foundation\AbstractServiceProvider;
+use Flarum\Http\Middleware\AuthenticateWithHeader;
+use Flarum\Http\Middleware\AuthenticateWithSession;
+use Flarum\Http\Middleware\DispatchRoute;
+use Flarum\Http\Middleware\ParseJsonBody;
+use Flarum\Http\Middleware\RememberFromCookie;
+use Flarum\Http\Middleware\SetLocale;
+use Flarum\Http\Middleware\StartSession;
 use Flarum\Http\RouteCollection;
 use Flarum\Http\RouteHandlerFactory;
 use Flarum\Http\UrlGenerator;
 use Tobscure\JsonApi\ErrorHandler;
 use Tobscure\JsonApi\Exception\Handler\FallbackExceptionHandler;
 use Tobscure\JsonApi\Exception\Handler\InvalidParameterExceptionHandler;
+use Zend\Stratigility\MiddlewarePipe;
 
 class ApiServiceProvider extends AbstractServiceProvider
 {
@@ -39,6 +50,27 @@ class ApiServiceProvider extends AbstractServiceProvider
             return new RouteCollection;
         });
 
+        $this->app->singleton('flarum.api.middleware', function ($app) {
+            $pipe = new MiddlewarePipe;
+            $pipe->raiseThrowables();
+
+            $pipe->pipe($app->make(HandleErrors::class));
+
+            $pipe->pipe($app->make(ParseJsonBody::class));
+            $pipe->pipe($app->make(FakeHttpMethods::class));
+            $pipe->pipe($app->make(StartSession::class));
+            $pipe->pipe($app->make(RememberFromCookie::class));
+            $pipe->pipe($app->make(AuthenticateWithSession::class));
+            $pipe->pipe($app->make(AuthenticateWithHeader::class));
+            $pipe->pipe($app->make(SetLocale::class));
+
+            event(new ConfigureMiddleware($pipe, 'api'));
+
+            $pipe->pipe($app->make(DispatchRoute::class, ['routes' => $app->make('flarum.api.routes')]));
+
+            return $pipe;
+        });
+
         $this->app->singleton(ErrorHandler::class, function () {
             $handler = new ErrorHandler;