Make notifications work for group-level work-items
Problem
When sending notifications for a work item, we must take group-level work items into account.
Right now, we depend on project in places like: https://gitlab.com/gitlab-org/gitlab/-/blob/3694ea7d549385b5022f69156f02e6d570477475/app/mailers/emails/issues.rb#L190 and https://gitlab.com/gitlab-org/gitlab/-/blob/3694ea7d549385b5022f69156f02e6d570477475/app/mailers/emails/issues.rb#L178-179
Proposal
Notifications should work the same for work items as they do for issues, no matter which Group or Project owns the work item.
Edited by Gabe Weaver