[GitLab] The repository input needs to be broadend

[nihalxkumar]

The current string format is username/repo.

While this works for many projects, it does not encompass the GitLab method of managing larger projects,
which follows the format of organization/project/subproject.

It also fails to accept when given the entire URL with https://gitlab.com/organization/project/subproject

[akostadinov]

Another example https://gitlab.com/gitlab-org/ruby/gems/prometheus-client-mmap

[mvanhorn]

Submitted the cache-key half of this in #522. The frontend regex already accepts subgroup URLs, so the user-visible failure is downstream of input parsing - PR body has a scope note on what's covered and what's next.