Allow cancelling running jobs via comment

[mfocko]

Apart from automatically cancelling the jobs, we should also allow users to cancel them manually (even without any push to the branch).

TODO

• Determine the Packit command to be used when cancelling the jobs via comment
• Consider the UX, we could require job Packit job IDs for cancelling, but also do the deduction automatically

Note

Deducing the most recently triggered jobs based on the comment is more time-consuming than directly knowing what to cancel.

Note

It may be complex to cancel jobs triggered by the comment. (caused by the structuring in the DB)