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/forum_topic.rb

@@ -22,6 +22,7 @@ class ForumTopic < ApplicationRecord
   has_many :bulk_update_requests
   has_many :tag_aliases
   has_many :tag_implications
+  has_many :mod_actions, as: :subject, dependent: :destroy
 
   validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
   validates :category_id, inclusion: { in: CATEGORIES.keys }
@@ -32,7 +33,7 @@ class ForumTopic < ApplicationRecord
   after_update :update_posts_on_deletion_or_undeletion
   after_update :update_original_post
   after_save(:if => ->(rec) {rec.is_locked? && rec.saved_change_to_is_locked?}) do |rec|
-    ModAction.log("locked forum topic ##{id} (title: #{title})", :forum_topic_lock)
+    ModAction.log("locked forum topic ##{id} (title: #{title})", :forum_topic_lock, subject: self, user: CurrentUser.user)
   end
 
   deletable
@@ -164,11 +165,11 @@ class ForumTopic < ApplicationRecord
   end
 
   def create_mod_action_for_delete
-    ModAction.log("deleted forum topic ##{id} (title: #{title})", :forum_topic_delete)
+    ModAction.log("deleted forum topic ##{id} (title: #{title})", :forum_topic_delete, subject: self, user: CurrentUser.user)
   end
 
   def create_mod_action_for_undelete
-    ModAction.log("undeleted forum topic ##{id} (title: #{title})", :forum_topic_undelete)
+    ModAction.log("undeleted forum topic ##{id} (title: #{title})", :forum_topic_undelete, subject: self, user: CurrentUser.user)
   end
 
   def page_for(post_id)