Allow specifying revs in repogroups

Created by: dadlerj

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

As a workaround to the end state discussed in RFC 136 (https://docs.google.com/document/d/1-Hn3bSVmwZ9iY0nfztXMoDfzWMdOHYTKrhK2F5ZJZI8/edit#)

The use case here is that they would like to add search scopes for searching across their code as of a given release version (e.g. v1.1, released 3 months ago).

This is possible today (using a release that contains the functionality in https://github.com/sourcegraph/sourcegraph/pull/9425), but it ends up with a VERY long and confusing term in the search bar, e.g.:

repo:^github\.com/sourcegraph/sourcegraph$@abc123|^github\.com/sourcegraph/about$@xyz789|^github\.com/sourcegraph/other-repo$@aaa111|^github\.com/sourcegraph/other-repo-2$@bbb222|^github\.com/sourcegraph/other-repo-3$@ccc333|...

In this user's case, the list of repos is ~15-20, so it's quite confusing for users.

Allowing repogroups to specify repos-at-revs (instead of just a list of repos) would make this much cleaner. That query fragment becomes repogroup:v1.1