Resolve "Use Danger gem"

What does this MR do?

This adds Danger to our pipelines to some automate merge request checks.

Are there points in the code the reviewer needs to double check?

1. This depends on https://github.com/danger/danger/pull/989 => Released as part of Danger v5.6.1.
2. Please give feedback on the checks, I suggest starting with not too many checks and iterate to add more in the future.
3. Check how that works for forks: we could add a webhook on MR creation that would trigger a pipeline that would only run a danger job... => https://gitlab.com/gitlab-org/gitlab-ce/issues/49134
4. We should add a check for number of commits behind master. If we're xxx (e.g. 200) commits behind master, the pipeline should fail => the only downside is that this check is only run on push, not when master is updated with new commits... => https://gitlab.com/gitlab-org/gitlab-ce/issues/49135

Why was this MR needed?

Because the current merge request checklist is tedious/heavy/not user-friendly, and should be automated as much as possible.

Screenshot

Does this MR meet the acceptance criteria?

• [-] API support added
• [-] Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #20700 (closed)