Add group level labels.

* `LabelsFinder`
* `Gitlab::Gfm::ReferenceRewriter`
* `Banzai::Filter::LabelReferenceFilter`

We'll be adding more feature that allow you to do cross-project management of issues.

loses #19997

See merge request !6425
Signed-off-by: Rémy Coutable <remy@rymai.me>