Repogroup autocomplete

Created by: dadlerj

Reported by https://app.hubspot.com/contacts/2762526/company/768958891

Unlike most search filters, the repogroup: filter doesn't provide any autocomplete options. It would be nice to pull them from settings and make this possible!

This is related to RFC-136 (which would solve the need for this user), but this would be beneficial on its own regardless, even if not a priority yet.