Allow user to revert back to non-ElasticSearch search
As part of &1736 we want to have a controlled rollout to groups on GitLab.com that opts them into the new ElasticSearch functionality. Since being opted into ElasticSearch means effectively having your ability to search with Postgres removed and may have some as yet undetermined bugs we'd rather avoid the need to rush to quickly (and manually) remove anyone in case they run into a specific bug that makes search less useful for them.
In general we don't want to keep having to urgently rollback our rollout every time we find a bug.
We should allow users the ability to search using the old Postgres search if they want from any of the search UIs.
I imagine that whenever you are on a search UI that is using ElasticSearch you can simply press a button (or toggle a checkbox) and your searches will happen as though ElasticSearch was not enabled.