Allow configuring cookie attributes

I decided to put this in config.php because if cookie settings were to
be stored in the database and configured via admin UI, entering
incorrect settings could cause the admin session to be destroyed,
requiring manual database intervention to fix. But it's a good prompt
for discussion as to which kind of settings belong in config.php vs the
database. Thoughts?

src/Http/Middleware/StartSession.php

@@ -22,13 +22,14 @@ use Zend\Stratigility\MiddlewareInterface;
 
 class StartSession implements MiddlewareInterface
 {
+    const COOKIE_NAME = 'session';
+
     /**
      * @var CookieFactory
      */
     protected $cookie;
 
     /**
-     * Rememberer constructor.
      * @param CookieFactory $cookie
      */
     public function __construct(CookieFactory $cookie)
@@ -56,7 +57,7 @@ class StartSession implements MiddlewareInterface
     {
         $session = new Session;
 
-        $session->setName('flarum_session');
+        $session->setName($this->cookie->getName(self::COOKIE_NAME));
         $session->start();
 
         if (! $session->has('csrf_token')) {
@@ -79,7 +80,7 @@ class StartSession implements MiddlewareInterface
     {
         return FigResponseCookies::set(
             $response,
-            $this->cookie->make($session->getName(), $session->getId())
+            $this->cookie->make(self::COOKIE_NAME, $session->getId())
         );
     }
 }