old_contents: from corehq.preindex import ExtraPreindexPlugin
  from django.conf import settings
  
  ExtraPreindexPlugin.register('domain', __file__, (
      settings.NEW_USERS_GROUPS_DB, settings.NEW_FIXTURES_DB, 'meta'))
  
new_contents: from corehq.preindex import ExtraPreindexPlugin
  from django.conf import settings
  
  ExtraPreindexPlugin.register('domain', __file__, (
      settings.NEW_DOMAINS_DB,
      settings.NEW_USERS_GROUPS_DB,
      settings.NEW_FIXTURES_DB,
      'meta',
  ))
  
subject: Add the new domains db