Add support for GitLab CI

[voelkle-unige]

closes #130

This PR adds support for GitLab CI.

A few choices I made:

• There is no automatic release feature. It could be done but requires a simple mechanism to bump the version.
• Instead of committing the updated catalogue and the badges to the repo, they are generated as CI artifacts. This makes the badge URLs absolute. Pushing to the repo would be more complicated for the user as it would require them to:
  1. generate a token with push rights
  2. create a CI variable with the token
• To avoid breaking the style of the existing actions.html page, I created a separate page.

Please let me know what you think.

[PonteIneptique]

I did not forget about it, I just need time :)