New user activity feed API.

Originally the user activity feed was implemented using UNIONs. I was
looking at make an API to add activity “sources”, or extra UNION
queries (select from posts, mentions, etc.) but quickly realised that
this is too slow and there’s no way to make it scale.

So I’ve implemented an API which is very similar to how notifications
work (see previous commit). The `activity` table is an aggregation of
stuff that happens, and it’s kept in sync by an ActivitySyncer which is
used whenever a post it created/edited/deleted, a user is
mentioned/unmentioned, etc.

Again, the API is very simple (see Core\Activity\PostedActivity +
Core\Handlers\Events\UserActivitySyncer)

src/Api/Serializers/ActivitySerializer.php

@@ -9,6 +9,17 @@ class ActivitySerializer extends BaseSerializer
      */
     protected $type = 'activity';
 
+    /**
+     * A map of activity types (key) to the serializer that should be used to
+     * output the activity's subject (value).
+     *
+     * @var array
+     */
+    public static $subjects = [
+        'posted' => 'Flarum\Api\Serializers\PostBasicSerializer',
+        'joined' => 'Flarum\Api\Serializers\UserBasicSerializer'
+    ];
+
     /**
      * Serialize attributes of an Activity model for JSON output.
      *
@@ -18,9 +29,7 @@ class ActivitySerializer extends BaseSerializer
     protected function attributes($activity)
     {
         $attributes = [
-            'id'   => ((int) $activity->id) ?: str_random(5),
             'contentType' => $activity->type,
-            'content' => json_encode($activity->data),
             'time' => $activity->time->toRFC3339String()
         ];
 
@@ -37,8 +46,10 @@ class ActivitySerializer extends BaseSerializer
         return $this->hasOne('Flarum\Api\Serializers\UserBasicSerializer');
     }
 
-    public function post()
+    public function subject()
     {
-        return $this->hasOne('Flarum\Api\Serializers\PostSerializer');
+        return $this->hasOne(function ($activity) {
+            return static::$subjects[$activity->type];
+        });
     }
 }