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

db/structure.sql

@@ -1202,7 +1202,8 @@ CREATE TABLE public.moderation_reports (
     model_type character varying NOT NULL,
     model_id bigint NOT NULL,
     creator_id integer NOT NULL,
-    reason text NOT NULL
+    reason text NOT NULL,
+    status integer DEFAULT 0 NOT NULL
 );
 
 
@@ -3847,6 +3848,13 @@ CREATE INDEX index_moderation_reports_on_creator_id ON public.moderation_reports
 CREATE INDEX index_moderation_reports_on_model_type_and_model_id ON public.moderation_reports USING btree (model_type, model_id);
 
 
+--
+-- Name: index_moderation_reports_on_status; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_moderation_reports_on_status ON public.moderation_reports USING btree (status);
+
+
 --
 -- Name: index_news_updates_on_created_at; Type: INDEX; Schema: public; Owner: -
 --
@@ -5688,6 +5696,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20220110171021'),
 ('20220110171022'),
 ('20220110171023'),
-('20220110171024');
+('20220110171024'),
+('20220120233850');