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.

test/unit/favorite_group_test.rb

@@ -27,7 +27,7 @@ class FavoriteGroupTest < ActiveSupport::TestCase
       @fav_group.add(@post)
       assert_equal([@post.id], @fav_group.post_ids)
 
-      @post.expunge!
+      @post.expunge!(create(:admin_user))
       assert_equal([], @fav_group.reload.post_ids)
     end
   end