Support sub-groups and sub-sub-groups on gitlab

[foot]

When the gitlab provider was intro'd we accidentally copy-pasted a constraint from the github provider that validates and rejects subgroups (makes sense for github, not gitlab)

https://github.com/fluxcd/go-git-providers/pull/51/files#diff-4404cc5156273f46bd72ef805e119d1ed8ff3486f164274ab32611fb99dd1452R202

You can workaround this check in code calling ggp with

    // Add the subgroups
    ref.Organization = strings.Join(append([]string{ref.Organization}, ref.SubOrganizations...), "/")
    ref.SubOrganizations = nil

before using the ref to do something. At least these methods work fine:

• GetRepo
• CreateBranch
• CreateCommit
• CreatePullRequest

We should

1. remove the check in the validation.
2. Either reflect the suborgs in the org as above, or take the suborgs into account when doing requests.