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/user_deletion_test.rb

@@ -70,6 +70,9 @@ class UserDeletionTest < ActiveSupport::TestCase
     should "generate a modaction" do
       @deletion.delete!
       assert_match(/deleted user ##{@user.id}/, ModAction.last.description)
+      assert_equal(@user, ModAction.last.subject)
+      assert_equal("user_delete", ModAction.last.category)
+      assert_equal(@deletion.deleter, ModAction.last.creator)
     end
 
     should "remove any favorites" do
@@ -90,7 +93,9 @@ class UserDeletionTest < ActiveSupport::TestCase
 
       @deletion.delete!
       assert_equal("user_#{@user.id}", @user.reload.name)
-      assert_equal(true, ModAction.exists?(description: "deleted user ##{@user.id}", creator: @deletion.deleter))
+      assert_equal("deleted user ##{@user.id}", ModAction.last.description)
+      assert_equal(@deletion.deleter, ModAction.last.creator)
+      assert_equal(@user, ModAction.last.subject)
     end
 
     should "not work for other users" do