migrate (some projects? everything?) from trac to gitlab

Having both Trac and GitLab for TPO is not desirable in the long term, both for maintenance and consistency across projects.

If GitLab is okay for people, we should consider migrating to it and turning off (or turning into a static website) this Trac instance.

This ticket explores the practicalities behind this project.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information