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/mod_action.rb

@@ -2,6 +2,7 @@
 
 class ModAction < ApplicationRecord
   belongs_to :creator, :class_name => "User"
+  belongs_to :subject, polymorphic: true, optional: true
 
   # ####DIVISIONS#####
   # Groups:     0-999
@@ -70,6 +71,10 @@ class ModAction < ApplicationRecord
     other: 2000,
   }
 
+  def self.model_types
+    %w[Artist Comment CommentVote ForumPost ForumTopic IpBan ModerationReport Pool Post PostVote Tag TagAlias TagImplication User UserFeedback]
+  end
+
   def self.visible(user)
     if user.is_moderator?
       all
@@ -79,7 +84,7 @@ class ModAction < ApplicationRecord
   end
 
   def self.search(params, current_user)
-    q = search_attributes(params, [:id, :created_at, :updated_at, :category, :description, :creator], current_user: current_user)
+    q = search_attributes(params, [:id, :created_at, :updated_at, :category, :description, :creator, :subject], current_user: current_user)
 
     case params[:order]
     when "created_at_asc"
@@ -95,8 +100,8 @@ class ModAction < ApplicationRecord
     self.class.categories[category]
   end
 
-  def self.log(desc, cat = :other, user = CurrentUser.user)
-    create(creator: user, description: desc, category: categories[cat])
+  def self.log(description, category, subject:, user:)
+    create!(description: description, category: category, subject: subject, creator: user)
   end
 
   def self.available_includes