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/moderation_report.rb

@@ -7,6 +7,7 @@ class ModerationReport < ApplicationRecord
 
   belongs_to :model, polymorphic: true
   belongs_to :creator, class_name: "User"
+  has_many :mod_actions, as: :subject, dependent: :destroy
 
   before_validation(on: :create) { model.lock! }
   validates :reason, presence: true
@@ -61,9 +62,9 @@ class ModerationReport < ApplicationRecord
     return unless saved_change_to_status? && status != :pending
 
     if handled?
-      ModAction.log("handled modreport ##{id}", :moderation_report_handled, updater)
+      ModAction.log("handled modreport ##{id}", :moderation_report_handled, subject: self, user: updater)
     elsif rejected?
-      ModAction.log("rejected modreport ##{id}", :moderation_report_rejected, updater)
+      ModAction.log("rejected modreport ##{id}", :moderation_report_rejected, subject: self, user: updater)
     end
   end