Add API to collect posts that were removed during an action

This is useful for both the Sticky and Categories extensions, where if
you sticky a discussion and then immediately unsticky it, or if you
move it to a category and then immediately move it back, the last
“activity” post will be removed.

framework/core/src/Core/Models/Discussion.php

@@ -49,6 +49,13 @@ class Discussion extends Model
      */
     public $addedPosts = [];
 
+    /**
+     * An array of posts that have been removed during this request.
+     *
+     * @var \Flarum\Core\Models\Post[]
+     */
+    public $removedPosts = [];
+
     /**
      * The user for which the state relationship should be loaded.
      *
@@ -166,6 +173,18 @@ class Discussion extends Model
         $this->addedPosts[] = $post;
     }
 
+    /**
+     * Specify that a post was removed from this discussion during this
+     * request for later retrieval.
+     *
+     * @param  \Flarum\Core\Models\Post  $post
+     * @return void
+     */
+    public function postWasRemoved(Post $post)
+    {
+        $this->removedPosts[] = $post->id;
+    }
+
     /**
      * Define the relationship with the discussion's posts.
      *