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/models/post_approval.rb

@@ -31,7 +31,7 @@ class PostApproval < ApplicationRecord
     post.appeals.pending.update!(status: :succeeded)
 
     post.update(approver: user, is_flagged: false, is_pending: false, is_deleted: false)
-    ModAction.log("undeleted post ##{post_id}", :post_undelete, user) if is_undeletion
+    ModAction.log("undeleted post ##{post_id}", :post_undelete, subject: post, user: user) if is_undeletion
 
     post.uploader.upload_limit.update_limit!(is_pending, true)
   end