Comments: Added back-end content reference handling

Also added archived property, to be added.

app/Activity/Models/Comment.php

@@ -19,6 +19,8 @@ use Illuminate\Database\Eloquent\Relations\MorphTo;
  * @property int      $entity_id
  * @property int      $created_by
  * @property int      $updated_by
+ * @property string   $content_ref
+ * @property bool     $archived
  */
 class Comment extends Model implements Loggable
 {