Merge branch 'fix-ci-background-migration' into 'master'

Add missing helpers in background migration

See merge request gitlab-org/gitlab!27483

db/migrate/20200311141943_insert_ci_pipeline_schedules_plan_limits.rb

 # frozen_string_literal: true
 
 class InsertCiPipelineSchedulesPlanLimits < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
   DOWNTIME = false
 
   def change