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/moderation_reports_controller_test.rb

@@ -127,6 +127,8 @@ class ModerationReportsControllerTest < ActionDispatch::IntegrationTest
         assert_equal("handled", report.reload.status)
         assert_equal(true, @user.dmails.received.exists?(from: User.system, title: "Thank you for reporting comment ##{@comment.id}"))
         assert_equal(true, ModAction.moderation_report_handled.where(creator: @mod).exists?)
+        assert_equal(report, ModAction.last.subject)
+        assert_equal(@mod, ModAction.last.creator)
       end
 
       should "allow a moderator to mark a moderation report as rejected" do
@@ -137,6 +139,8 @@ class ModerationReportsControllerTest < ActionDispatch::IntegrationTest
         assert_equal("rejected", report.reload.status)
         assert_equal(false, @user.dmails.received.exists?(from: User.system))
         assert_equal(true, ModAction.moderation_report_rejected.where(creator: @mod).exists?)
+        assert_equal(report, ModAction.last.subject)
+        assert_equal(@mod, ModAction.last.creator)
       end
     end
   end