[CI] Add Danger to perform automated MR checks

With our new and stricter guidelines on how we want our commits and MRs to look, we should find a way to enforce this. GitLab uses Danger for this and it seems that this provides a suitable solution for us as well. I will look into adding this to our CI and see what checks are useful for us to have.

Useful links:

• https://docs.gitlab.com/development/dangerbot/
• https://github.com/danger/danger
• https://gitlab.com/gitlab-org/ruby/gems/gitlab-dangerfiles