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.

app/models/post_vote.rb

@@ -5,6 +5,7 @@ class PostVote < ApplicationRecord
 
   belongs_to :post
   belongs_to :user
+  has_many :mod_actions, as: :subject, dependent: :destroy
 
   validates :score, inclusion: { in: [1, -1], message: "must be 1 or -1" }
 
@@ -81,9 +82,9 @@ class PostVote < ApplicationRecord
     return if new_record? || updater.nil? || updater == user
 
     if is_deleted_changed?(from: false, to: true)
-      ModAction.log("deleted post vote ##{id} on post ##{post_id}", :post_vote_delete, updater)
+      ModAction.log("deleted post vote ##{id} on post ##{post_id}", :post_vote_delete, subject: self, user: updater)
     elsif is_deleted_changed?(from: true, to: false)
-      ModAction.log("undeleted post vote ##{id} on post ##{post_id}", :post_vote_undelete, updater)
+      ModAction.log("undeleted post vote ##{id} on post ##{post_id}", :post_vote_undelete, subject: self, user: updater)
     end
   end