Rewrite `create-sibling-gitlab`

[mih]

Various issues have been identified recently (list pending). The present implementation is old and spaghetti-ish. It would benefit from better integration with configuration and credential handling. Most of all it needs test that run against a real gitlab instance. We have a tester account on gitlab.com -- I have no idea why that is not used already.

Issues

• https://github.com/datalad/datalad/issues/6681
• https://github.com/datalad/datalad/issues/7246 (hints at the possible utility of a dedicated extension, as is https://github.com/datalad/datalad/issues/7025)
• https://github.com/datalad/datalad/issues/5242
• https://github.com/datalad/datalad/issues/6012
• https://github.com/datalad/datalad/issues/3467
• https://github.com/datalad/datalad/issues/7409

[mih]

datalad-next now carries a patch in this direction and already has pytest-ified tests.