Apply labels to PR

[mdellweg]

This labels the pr on some conditions. For example if there is a WIP keyword in any or a changelog entry in every commit.

Review Checklist:

• [ ] An issue is properly linked. [feature and bugfix only]
• [ ] Tests are present or not feasible.
• [ ] Commits are split in a logical way (not historically).

[mdellweg]

This should help us get away from writing "[noissue]" in any commits.

(This never really worked anyway, because Github dos not want reviewers to look at commits.)