mod actions: record the subject of the mod action.

Add a polymorphic `subject` field that records the subject of the mod
action. The subject is the post, user, comment, artist, etc the mod
action is for.

* The subject for the user ban and unban actions is the user, not the ban itself.
* The subject for the user feedback update and deletion actions is the user,
  not the feedback itself.
* The subject for the post undeletion action is the post, not the approval itself.
* The subject for the move favorites action is the source post where the
  favorites were moved from, not the destination post where the favorites
  were moved to.
* The subject for the post permanent delete action is nil, because the
  post itself is hard deleted.
* When a post is permanently deleted, all mod actions related to the
  post are deleted as well.

app/logical/user_deletion.rb

@@ -42,7 +42,7 @@ class UserDeletion
   private
 
   def create_mod_action
-    ModAction.log("deleted user ##{user.id}", :user_delete, deleter)
+    ModAction.log("deleted user ##{user.id}", :user_delete, subject: user, user: deleter)
   end
 
   def create_user_event