In order to allow us to incrementally enable ES on gitlab.com we add two
 new classes:

- ElasticsearchIndexedNamespaces
- ElasticsearchIndexedProjects

These classes are used by ApplicationSetting to enable/disable projects
and namespaces (groups) that should be indexed by elasticsearch.

We also have the application setting, `elasticsearch_limit_indexing`,
that enables/disables the new functionality

In order to be able to selectively choose projects/namespaces to use
with elasticsearch, `elasticsearch_limit_indexing` MUST be enabled under
the admin integrations options.