Single CI/CD pipeline definitions

With this MR, we replace our per-subproject CI/CD pipeline definitions by a central pipeline for all subprojects. The main reason for this is that triggering child pipelines does not work as intended. We always had issues with failing pipelines if changes in merge requests were not successfully captures and also the visualization for parent/child pipelines is bad. It is likely that at some point we might switch back to individual pipelines per subproject, but for now this solutions seems to be best in relation to effort.

This MR also does:

• Closes #51 (closed)
• Closes #124 (closed)