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 30 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Lint
passed #86055
lint_black

00:00:30

passed #86056
lint_flake8

00:00:19