Fix `downtime_check` job

15fcd965 (!18547) squashes old migrations
to a single one.

Old migrations had `rubocop:disable all` rule, but the new migration doesn't
have it, so the `downtime_check` fails.

This commit explicitly tells that the migration requires downtime.

db/migrate/20140313092127_init_schema.rb

 class InitSchema < ActiveRecord::Migration
+  DOWNTIME = true
+
   def up
     create_table "broadcast_messages", force: :cascade do |t|
       t.text "message", null: false