Refactor CoreServiceProvider

A good start I think, but still some work to do. If we go ahead with
https://github.com/flarum/core/issues/132#issuecomment-117507974 (which
I am in favour of), we can extract the entity-related stuff into some
smaller service providers (e.g. discussion repo, an event listener,
permissions, and gambits stuff could all go in
Flarum\Core\Discussions\DiscussionsServiceProvider).

framework/core/src/Extend/ActivityType.php

@@ -10,18 +10,27 @@ class ActivityType implements ExtenderInterface
 
     protected $serializer;
 
-    public function __construct($class, $serializer)
+    public function __construct($class)
     {
         $this->class = $class;
+    }
+
+    public function subjectSerializer($serializer)
+    {
         $this->serializer = $serializer;
+
+        return $this;
     }
 
     public function extend(Container $container)
     {
         $class = $this->class;
+        $type = $class::getType();
 
-        Activity::setSubjectModel($class::getType(), $class::getSubjectModel());
+        Activity::setSubjectModel($type, $class::getSubjectModel());
 
-        ActivitySerializer::$subjects[$class::getType()] = $this->serializer;
+        if ($this->serializer) {
+            ActivitySerializer::setSubjectSerializer($type, $this->serializer);
+        }
     }
 }

framework/core/src/Extend/DiscussionGambit.php

@@ -13,8 +13,6 @@ class DiscussionGambit implements ExtenderInterface
 
     public function extend(Container $container)
     {
-        $container->make('events')->listen('Flarum\Core\Events\RegisterDiscussionGambits', function ($event) {
-            $event->gambits->add($this->class);
-        });
+        $container->make('flarum.discussionGambits')[] = $this->class;
     }
 }

framework/core/src/Extend/NotificationType.php

@@ -13,10 +13,16 @@ class NotificationType implements ExtenderInterface
 
     protected $enabled = [];
 
-    public function __construct($class, $serializer)
+    public function __construct($class)
     {
         $this->class = $class;
+    }
+
+    public function subjectSerializer($serializer)
+    {
         $this->serializer = $serializer;
+
+        return $this;
     }
 
     public function enableByDefault($method)