Subsystem maintainer permissions on GitLab

Problem/Motivation

With the adoption of gitlab there a few challenges and oppertunities to have subsystem maintainers help out core committers with certian tasks.

Things like retargeting or closing merge requests. This is only possible for accounts with permissions on the drupal project.

This requires is being maintainer or developer. The issue with that is that this would give commit permissions. But we can fix that with branch permissions

https://docs.gitlab.com/ee/user/project/protected_branches.html

By protecting the release branches we can give more permissions to subsystem maintainers but also protect branches from commits by us.

Steps to reproduce

Proposed resolution

Setup branch permissons.
Add subsystem maintainers to project/drupal.

Remaining tasks

1. Collect subsystem maintainer usernames
2. Setup branch permissions
3. Test with single user
4. Add all subsystem maintainers

User interface changes

API changes

Data model changes

Release notes snippet