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/comment_votes_controller_test.rb

@@ -214,7 +214,8 @@ class CommentVotesControllerTest < ActionDispatch::IntegrationTest
         assert_equal(1, @vote.comment.score)
 
         assert_difference("CommentVote.count", 0) do
-          delete_auth comment_vote_path(@vote), create(:admin_user), xhr: true, params: { variant: "listing" }
+          admin = create(:admin_user)
+          delete_auth comment_vote_path(@vote), admin, xhr: true, params: { variant: "listing" }
 
           assert_response :success
           assert_equal(true, @vote.reload.is_deleted?)
@@ -222,6 +223,8 @@ class CommentVotesControllerTest < ActionDispatch::IntegrationTest
 
           assert_equal("comment_vote_delete", ModAction.last.category)
           assert_match(/deleted comment vote/, ModAction.last.description)
+          assert_equal(@vote, ModAction.last.subject)
+          assert_equal(admin, ModAction.last.creator)
         end
       end