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/ip_bans_controller_test.rb

@@ -28,6 +28,8 @@ class IpBansControllerTest < ActionDispatch::IntegrationTest
 
         assert_equal("ip_ban_create", ModAction.last&.category)
         assert_match(/created ip ban for 1\.2\.3\.4/, ModAction.last.description)
+        assert_equal(IpBan.last, ModAction.last.subject)
+        assert_equal(@admin, ModAction.last.creator)
       end
     end
 
@@ -65,6 +67,8 @@ class IpBansControllerTest < ActionDispatch::IntegrationTest
         assert_equal(true, @ip_ban.reload.is_deleted)
         assert_equal("ip_ban_delete", ModAction.last.category)
         assert_match(/deleted ip ban for #{@ip_ban.ip_addr}/, ModAction.last.description)
+        assert_equal(@ip_ban, ModAction.last.subject)
+        assert_equal(@admin, ModAction.last.creator)
       end
 
       should "mark an ip ban as undeleted" do
@@ -75,6 +79,8 @@ class IpBansControllerTest < ActionDispatch::IntegrationTest
         assert_equal(false, @ip_ban.reload.is_deleted?)
         assert_equal("ip_ban_undelete", ModAction.last.category)
         assert_match(/undeleted ip ban for #{@ip_ban.ip_addr}/, ModAction.last.description)
+        assert_equal(@ip_ban, ModAction.last.subject)
+        assert_equal(@admin, ModAction.last.creator)
       end
     end
   end