Automatically create a repo on lab.c.o when a new extension is added on c.o

Recap of some online or Barcelona-2019 discussions:

Currently to create new extensions on Gitlab, we have to ping @bgm on Mattermost. This was meant to be temporary until we find a better procedure (and validate that we want to use Gitlab).
While Gitlab projects can have tags/labels and there is a search, it's not sufficiently user-friendly for most users. It's unlikely to ever replace the official extension directory on our website.
We want to encourage using Gitlab:
- Grow our developer community, as a community, i.e. encourage collaboration between developers/shops, reduce shop-centric development silos.
- More user-friendly for CiviCRM users, who are more likely to have a Gitlab/civicrm.org account than a Github account.

Things we can improve:

Make sure that creating new extensions is as simple as possible (some unused fields have been removed as part of the Drupal8 migration).
When we create a new extension on civicrm.org, automatically create a Gitlab repository
Automatically set the description of the project on Gitlab.
We could eventually do things such as setup a webhook to run extdir-scan when a new tag/release is pushed.