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/comment.rb

@@ -6,11 +6,13 @@ class Comment < ApplicationRecord
   belongs_to_updater
 
   has_many :moderation_reports, as: :model, dependent: :destroy
+  has_many :pending_moderation_reports, -> { pending }, as: :model, class_name: "ModerationReport"
   has_many :votes, class_name: "CommentVote", dependent: :destroy
 
   validates :body, presence: true, length: { maximum: 15_000 }, if: :body_changed?
 
   before_create :autoreport_spam
+  before_save :handle_reports_on_deletion
   after_create :update_last_commented_at_on_create
   after_update(:if => ->(rec) {(!rec.is_deleted? || !rec.saved_change_to_is_deleted?) && CurrentUser.id != rec.creator_id}) do |rec|
     ModAction.log("comment ##{rec.id} updated by #{CurrentUser.user.name}", :comment_update)
@@ -78,6 +80,13 @@ class Comment < ApplicationRecord
     end
   end
 
+  def handle_reports_on_deletion
+    return unless Pundit.policy!(updater, ModerationReport).update?
+    return unless moderation_reports.pending.present? && is_deleted_change == [false, true]
+
+    moderation_reports.pending.update!(status: :handled)
+  end
+
   def quoted_response
     DText.quote(body, creator.name)
   end