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.
* Add ability to mark moderation reports as 'handled' or 'rejected'.
* Automatically mark reports as handled when the comment or forum post
is deleted.
* Send a dmail to the reporter when their report is handled.
* Don't show the report notice on comments or forum posts when all
reports against it have been handled or rejected.
* Add a fix script to mark all existing reports for deleted comments,
forum posts, or dmails as handled.
