Include the assignee in the list of people notified on a ticket/PR
We always include the assignee in the notifications (even on private
tickets), but so far we did not show them in the list of subscriber.
With this commit, this oversight is now fixed.
Fixes https://pagure.io/pagure/issue/4957
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>