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/pool.rb

@@ -14,6 +14,8 @@ class Pool < ApplicationRecord
   before_validation :normalize_name
   after_save :create_version
 
+  has_many :mod_actions, as: :subject, dependent: :destroy
+
   deletable
   has_dtext_links :description
 
@@ -157,11 +159,11 @@ class Pool < ApplicationRecord
   end
 
   def create_mod_action_for_delete
-    ModAction.log("deleted pool ##{id} (name: #{name})", :pool_delete)
+    ModAction.log("deleted pool ##{id} (name: #{name})", :pool_delete, subject: self, user: CurrentUser.user)
   end
 
   def create_mod_action_for_undelete
-    ModAction.log("undeleted pool ##{id} (name: #{name})", :pool_undelete)
+    ModAction.log("undeleted pool ##{id} (name: #{name})", :pool_undelete, subject: self, user: CurrentUser.user)
   end
 
   def add!(post)