[Isbg] Migrating to Gitlab

Hey folks,

I'd like to migrate to Gitlab. If you have an opinion on this, I'd like to hear it:

https://github.com/isbg/isbg/issues/131

If on November 19th 2019 (in 2 months) we don't have a strong consensus on the issue tracker against moving to Gitlab, I'll initiate the move.

Cheers!