Clean up CI definitions
The CI definitions are meanwhile quite dated and new features in Gitlab are available. We shall make use of them and:
- Use job inheritance to avoid spawning pipelines e.g. when importing translations.
- Migrate to
rules
instead ofonly
/except
. - In addition, we ideally remove the image builds and create dedicated projects to build Docker images for the CI (could be shared with other projects as well, as qlmdb and SynqClient).