Index projects on repository storage

What does this MR do?

Indexes projects by repository storage

Are there points in the code the reviewer needs to double check?

I've got this in CE to avoid schema drift between the two versions. I can see it being useful in CE in its own right though - it would allow us to show stats like the number of projects per shard more efficiently, for instance.

Why was this MR needed?

Improve performance of some SQL queries.

Discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2974#note_40910135

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/3360 https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2974