Fix #4669: Track moderation report status.

* Add ability to mark moderation reports as 'handled' or 'rejected'.
* Automatically mark reports as handled when the comment or forum post
  is deleted.
* Send a dmail to the reporter when their report is handled.
* Don't show the report notice on comments or forum posts when all
  reports against it have been handled or rejected.
* Add a fix script to mark all existing reports for deleted comments,
  forum posts, or dmails as handled.

app/policies/moderation_report_policy.rb

@@ -13,11 +13,19 @@ class ModerationReportPolicy < ApplicationPolicy
     unbanned? && policy(record.model).reportable?
   end
 
+  def update?
+    user.is_moderator?
+  end
+
   def can_see_moderation_reports?
     user.is_moderator?
   end
 
-  def permitted_attributes
+  def permitted_attributes_for_create
     [:model_type, :model_id, :reason]
   end
+
+  def permitted_attributes_for_update
+    [:status]
+  end
 end