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/models/forum_post.rb

@@ -8,6 +8,7 @@ class ForumPost < ApplicationRecord
   belongs_to :topic, class_name: "ForumTopic", inverse_of: :forum_posts
 
   has_many :moderation_reports, as: :model
+  has_many :pending_moderation_reports, -> { pending }, as: :model, class_name: "ModerationReport"
   has_many :votes, class_name: "ForumPostVote"
   has_one :tag_alias
   has_one :tag_implication
@@ -16,6 +17,7 @@ class ForumPost < ApplicationRecord
   validates :body, presence: true, length: { maximum: 200_000 }, if: :body_changed?
 
   before_create :autoreport_spam
+  before_save :handle_reports_on_deletion
   after_create :update_topic_updated_at_on_create
   after_update :update_topic_updated_at_on_update_for_original_posts
   after_destroy :update_topic_updated_at_on_destroy
@@ -157,6 +159,12 @@ class ForumPost < ApplicationRecord
     end
   end
 
+  def handle_reports_on_deletion
+    return unless moderation_reports.pending.present? && is_deleted_change == [false, true]
+
+    moderation_reports.pending.update!(status: :handled)
+  end
+
   def async_send_discord_notification
     DiscordNotificationJob.perform_later(forum_post: self)
   end