Add latest changes from gitlab-org/gitlab@master

changelogs/unreleased/13401-close-gitlab-issue-on-recovery-alerts-from-prometheus.yml

+---
+title: Close issues on Prometheus alert recovery
+merge_request: 18431
+author:
+type: added

db/migrate/20191003015155_add_self_managed_prometheus_alerts.rb

 # frozen_string_literal: true
 class AddSelfManagedPrometheusAlerts < ActiveRecord::Migration[5.2]
-  # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
   def change