CI: Change to Direct Acyclic Graph to Run Jobs in Parallel

This change makes use of the DAG feature of GitLab 12.something which allows to already start jobs later in the pipeline of they have no dependencies.

For us this allows to right away start the linting process, which takes longest. The deploy stage is left without the needs keyword and will therefore only be processed once all previous jobs from the DAG have successfully finished.

This cuts down a typical MR pipeline from ~1h runtime to ~35min.