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/user.rb

@@ -139,6 +139,7 @@ class User < ApplicationRecord
   has_many :tag_implications, foreign_key: :creator_id
   has_many :uploads, foreign_key: :uploader_id, dependent: :destroy
   has_many :upload_media_assets, through: :uploads, dependent: :destroy
+  has_many :mod_actions, as: :subject, dependent: :destroy
   belongs_to :inviter, class_name: "User", optional: true
 
   accepts_nested_attributes_for :email_address, reject_if: :all_blank, allow_destroy: true