GitLab CI Yaml template dropdown

What does this MR do?

Make it possible to select a dropdown for an easy start with GitLab CI.

What are the relevant issue numbers?

Closes #17521 (closed)

TODO

• Backend
  • CHANGELOG item
  • Fix rubocop failure
  • API Support
  • New tests
  • Add disclaimer to the top of the gitlab-ci.yml
• Frontend
  • New tests