repositories graphql resolver should allow for regex search

Created by: slimsag

It has the ability to do so on the backend, we just don't expose it. It would be nice when trying to query repositories on Sourcegraph selectively (such as all repos from multiple orgs).

This is also useful when trying to list repositories programmatically and get back repositories only ending in e.g. example excluding example-foobar

Requested by https://app.hubspot.com/contacts/2762526/company/407948923