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/Event/ConfigureMiddleware.php

@@ -11,10 +11,6 @@
 
 namespace Flarum\Event;
 
-use Flarum\Admin\Server as AdminServer;
-use Flarum\Api\Server as ApiServer;
-use Flarum\Forum\Server as ForumServer;
-use Flarum\Foundation\AbstractServer;
 use Zend\Stratigility\MiddlewarePipe;
 
 class ConfigureMiddleware
@@ -27,42 +23,35 @@ class ConfigureMiddleware
     /**
      * @var string
      */
-    public $path;
-
-    /**
-     * @var AbstractServer
-     */
-    public $server;
+    public $stackName;
 
     /**
      * @param MiddlewarePipe $pipe
-     * @param string $path
-     * @param AbstractServer $server
+     * @param string $stackName
      */
-    public function __construct(MiddlewarePipe $pipe, $path, AbstractServer $server)
+    public function __construct(MiddlewarePipe $pipe, $stackName)
     {
         $this->pipe = $pipe;
-        $this->path = $path;
-        $this->server = $server;
+        $this->stackName = $stackName;
     }
 
     public function pipe(callable $middleware)
     {
-        $this->pipe->pipe($this->path, $middleware);
+        $this->pipe->pipe($middleware);
     }
 
     public function isForum()
     {
-        return $this->server instanceof ForumServer;
+        return $this->stackName === 'forum';
     }
 
     public function isAdmin()
     {
-        return $this->server instanceof AdminServer;
+        return $this->stackName === 'admin';
     }
 
     public function isApi()
     {
-        return $this->server instanceof ApiServer;
+        return $this->stackName === 'api';
     }
 }