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.
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