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/ban.rb

@@ -81,11 +81,11 @@ class Ban < ApplicationRecord
   end
 
   def create_ban_mod_action
-    ModAction.log(%{banned <@#{user_name}> #{humanized_duration}: #{reason}}, :user_ban, banner)
+    ModAction.log(%{banned <@#{user_name}> #{humanized_duration}: #{reason}}, :user_ban, subject: user, user: banner)
   end
 
   def create_unban_mod_action
-    ModAction.log(%{unbanned <@#{user_name}>}, :user_unban)
+    ModAction.log(%{unbanned <@#{user_name}>}, :user_unban, subject: user, user: CurrentUser.user)
   end
 
   def self.available_includes