Notification emails regarding comments not threaded properly with other types of notifications

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29889 makes notifications about comments always have a @group.

We have https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/mailers/notify.rb#L116 which adds the group name to the subject if @group is present.

Other notifications like changing labels do not have the group name in the subject.

Emails with different subjects are not threaded in GMail even if we set the right headers.

To fix this, I think we should also do @group = @project.try(:group) for other notification types?