new push rule: reject commit messages based on negative match regex

What does this MR do?

This MR creates a new push rule Commit Messages (negative match).
Every commit message that matches the regex provided in it, will be rejected.

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

Why was this MR needed?

RE2 doesn't allow negative look ahead regexes.
This is a regression, because this was possible before switching to RE2.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/5743

/cc @jamedjo