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/moderator/post/posts_controller_test.rb

@@ -69,7 +69,7 @@ module Moderator
 
         context "unban action" do
           should "render" do
-            @post.ban!
+            @post.ban!(@admin)
             post_auth unban_moderator_post_post_path(@post), @admin
 
             assert_redirected_to(@post)