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/functional/post_votes_controller_test.rb

@@ -287,6 +287,8 @@ class PostVotesControllerTest < ActionDispatch::IntegrationTest
         assert_equal(0, @post.votes.active.count)
         assert_equal(true, @vote.reload.is_deleted?)
         assert_match(/deleted post vote #\d+ on post #\d+/, ModAction.post_vote_delete.last.description)
+        assert_equal(@vote, ModAction.last.subject)
+        assert_equal(admin, ModAction.last.creator)
       end
 
       should "not fail when attempting to remove an already removed vote" do