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/post_approval_test.rb

@@ -68,6 +68,8 @@ class PostApprovalTest < ActiveSupport::TestCase
           assert_equal(true, @post.reload.is_active?)
           assert_equal("post_undelete", ModAction.last.category)
           assert_equal("undeleted post ##{@post.id}", ModAction.last.description)
+          assert_equal(@post, ModAction.last.subject)
+          assert_equal(@new_approver, ModAction.last.creator)
         end
       end