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
## TODO
- [ ] Backend
- [x] CHANGELOG item
- [x] Fix rubocop failure
- [x] API Support
- [x] New tests
- [x] Add disclaimer to the top of the gitlab-ci.yml
- [ ] Frontend
- [x] New tests
See merge request !4411