feat(notification): add comment on “old” MR to notify the assignees there is pending work

Only old MR (for now 10 days without activity), which are not wip, not client
related, and with an assignee, add a friendly comment.

closes: cubicweb/cubicweb#101
2 jobs for topic/default/notification in 33 seconds (queued for 18 seconds)
Status Name Job ID Coverage
passed lint_black #86066


passed lint_flake8 #86067