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.

test/factories/moderation_report.rb

@@ -2,5 +2,6 @@ FactoryBot.define do
   factory(:moderation_report) do
     creator
     reason {"xxx"}
+    status { :pending }
   end
 end