Implement retriggering the downstream jobs via dist-git pull-request comments

[lbarcziova]

Implement retriggering downstream Koji build and Bodhi update jobs via dist-git pull-request comments related to the dist-git commit:
TODO:

• Implement retriggering downstream Koji build via dist-git pull-request comments #1540
• Implement retriggering Bodhi updates via dist-git pull-request comments #1541

[stale]

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[lbarcziova]

The subtasks are implemented, closing.