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/bans_controller_test.rb

@@ -61,6 +61,8 @@ class BansControllerTest < ActionDispatch::IntegrationTest
           assert_redirected_to bans_path
           assert_equal(true, @user.reload.is_banned?)
           assert_match(/banned <@#{@user.name}> 1 day: xxx/, ModAction.last.description)
+          assert_equal(@user, ModAction.last.subject)
+          assert_equal(@mod, ModAction.last.creator)
         end
       end
 
@@ -135,6 +137,8 @@ class BansControllerTest < ActionDispatch::IntegrationTest
 
           assert_redirected_to bans_path
           assert_match(/unbanned <@#{@ban.user.name}>/, ModAction.last.description)
+          assert_equal(@ban.user, ModAction.last.subject)
+          assert_equal(@mod, ModAction.last.creator)
         end
       end
     end