CI/CD releases on GitLab

Follow https://archives.docs.gitlab.com/14.10/ee/user/project/releases/#use-a-generic-package-for-attaching-binaries and automate creating releases when building a version that has a release tag. Thusly gain:

• Automated release entries on Whetstone
• Proper uploading of releases' binary assets (seems only that makes them not be deleted later if I interpret the link above correctly)
• Those will then have predictable permanent links, which gives us (i.a):
• The ability to link to these assets from F-Droid and have reproducible builds with our own signing keys
• Knowledge of how to do this right, since this is something we might want to do for our other software as well (PrivateStorage Desktop comes to mind)