Progressively enable elasticsearch for indexed projects
Currently on GitLab.com, we're engaging in backfill to get all our projects indexed. Until that's complete, the ES index we're building is expensive and unused. We can't even evalute its usefulness until the whole process is completed.
For project search, we could switch between ordinary or elasticsearch depending on whether the project has been indexed or not. We have some data for that in the
index_status table. All we need to add is tracking for wiki and database indexing (perhaps simple booleans in that table).
Conceivably, we could extend this to group search as well ("use elasticsearch if all projects in this group are indexed, otherwise normal search"), but application search would still need to be manually enabled / disabled.