WIP: Add CI config handling in project overview
Add handling for .gitlab-ci.yml similar to licenses and contribution guides. When there's a .gitlab-ci.yml present, add a button to go to "CI config". When it isn't present, add "Add CI config".
Related to #17521 (closed).
[I didn't add any tests, and I haven't tested this extensively, especially caching and responsiveness. Feel free to ignore and close this MR. I was just using it as an excuse to get to know the code a bit better.]