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/comment_test.rb

@@ -159,9 +159,12 @@ class CommentTest < ActiveSupport::TestCase
         end
 
         should "create a mod action" do
-          assert_difference("ModAction.count") do
-            @comment.update(body: "nope")
-          end
+          @comment.update(body: "nope")
+
+          assert_equal(1, ModAction.count)
+          assert_equal("comment_update", ModAction.last.category)
+          assert_equal(@comment, ModAction.last.subject)
+          assert_equal(@mod, ModAction.last.creator)
         end
 
         should "credit the moderator as the updater" do