Allow more contributors to propose changes to our GitLab configuration

Currently our gitlab-config.git repo is only accessible to sysadmins.

IMO at least committers-team should be allowed to clone, and propose MRs against, that repo. This would address a number of problems:

• RMs could open MRs for group milestones changes they have to do (see b170e128).
• Committers could propose new group labels: in the last few days, 2 tails/tails project labels were created (because committers are allowed to do that, but they can't edit group labels) while these should really have been group labels.
• When a team onboards a new member or offboards a previous member, perhaps they could send a MR for the corresponding GitLab group membership change.
• Committers could propose new projects under the tails/ namespace, ensuring our common project settings & permission model are applied.

I propose we:

• migrate gitlab-config.git to GitLab
• allow committers to clone that repo and push to non-protected branches
• document how to propose changes to our GitLab config

I volunteer to do all this once we agree on something.