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/user_feedbacks_controller_test.rb

@@ -125,6 +125,8 @@ class UserFeedbacksControllerTest < ActionDispatch::IntegrationTest
           assert_redirected_to @user_feedback
           assert_equal("blah", @user_feedback.reload.body)
           assert_match(/updated user feedback for "#{@user.name}":\/users\/#{@user.id}/, ModAction.last.description)
+          assert_equal(@user, ModAction.last.subject)
+          assert_equal(@mod, ModAction.last.creator)
         end
 
         should "allow deleting feedbacks given to other users" do
@@ -133,6 +135,8 @@ class UserFeedbacksControllerTest < ActionDispatch::IntegrationTest
           assert_redirected_to @user_feedback
           assert(@user_feedback.reload.is_deleted?)
           assert_match(/updated user feedback for "#{@user.name}":\/users\/#{@user.id}/, ModAction.last.description)
+          assert_equal(@user, ModAction.last.subject)
+          assert_equal(@mod, ModAction.last.creator)
         end
 
         should "not allow updating feedbacks given to themselves" do