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/Api/ApiServiceProvider.php

@@ -22,6 +22,8 @@ class ApiServiceProvider extends ServiceProvider
                 return new UrlGenerator($this->app->make('flarum.api.routes'));
             }
         );
+
+        $this->app->register('Flarum\Locale\LocaleServiceProvider');
     }
 
     /**

framework/core/src/Api/Serializers/ActivitySerializer.php

@@ -3,9 +3,7 @@
 class ActivitySerializer extends BaseSerializer
 {
     /**
-     * The resource type.
-     *
-     * @var string
+     * @inheritdoc
      */
     protected $type = 'activity';
 
@@ -15,10 +13,7 @@ class ActivitySerializer extends BaseSerializer
      *
      * @var array
      */
-    public static $subjects = [
-        'posted' => 'Flarum\Api\Serializers\PostBasicSerializer',
-        'joined' => 'Flarum\Api\Serializers\UserBasicSerializer'
-    ];
+    protected static $subjectSerializers = [];
 
     /**
      * Serialize attributes of an Activity model for JSON output.
@@ -52,4 +47,9 @@ class ActivitySerializer extends BaseSerializer
             return static::$subjects[$activity->type];
         });
     }
+
+    public static function setSubjectSerializer($type, $serializer)
+    {
+        static::$subjectSerializers[$type] = $serializer;
+    }
 }