FEATURE: New 'Reviewable' model to make reviewable items generic

Includes support for flags, reviewable users and queued posts, with REST API
backwards compatibility.

Co-Authored-By: romanrizzi <romanalejandro@gmail.com>
Co-Authored-By: jjaffeux <j.jaffeux@gmail.com>

lib/guardian/post_guardian.rb

@@ -192,10 +192,11 @@ module PostGuardian
   end
 
   def can_delete_post_action?(post_action)
-    # You can only undo your own actions
-    is_my_own?(post_action) && not(post_action.is_private_message?) &&
+    return false unless is_my_own?(post_action) && !post_action.is_private_message?
+
+    # Bookmarks do not have a time constraint
+    return true if post_action.is_bookmark?
 
-    # Make sure they want to delete it within the window
     post_action.created_at > SiteSetting.post_undo_action_window_mins.minutes.ago
   end