Turn STABLE_BRANCH_CHECKLIST.md into a script

[fingolfin]

We should turn STABLE_BRANCH_CHECKLIST.md a script, to reduce the work and the risk of mistakes.

The classical trick to do this is to first create a script which just prints each step and asks "Have you completed this step? Y/N". Then gradually convert steps into actual commands.

To be able to test the script, it should allow specifying a fork of GAP (e.g. via an optional command line argument) on which it should operate. (Possibly might need forks of stuff in gap-infra and gap-s

For some of the steps, it might be convenient to use https://cli.github.com. This should help with creating milestones and issue labels.