Refactor no_merges handler into the check_commits super-handler

[Urgau]

This PR refactors the no_merges handler into the check_commits super-handler.

In the process I introduced to the check_commits super-handler:

• handling of labels (add & remove)
• handling of draft/rollup PRs

Step 3-ish of the plan in #1901 (comment)

no_merges is now ridiculously simple and short compared to before. Our efforts are paying of big!

[Urgau]

The warning will look like this.

---

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • 9cc6dce

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/{repository_name}.git {default_branch}
  $ git push --force-with-lease
  

[Kobzol]

Nice! Do you think you could write a simple test for this please? 😇

[Kobzol]

Awesome, thanks!