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.

db/migrate/20220120233850_add_status_to_moderation_reports.rb

@@ -0,0 +1,6 @@
+class AddStatusToModerationReports < ActiveRecord::Migration[7.0]
+  def change
+    add_column :moderation_reports, :status, :integer, null: false, default: 0
+    add_index :moderation_reports, :status
+  end
+end