Pipeline emails don't respect group notification emails

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25299 added group-level notification emails, so you can choose a different notification email for each group.

Pipeline emails don't use this setting, because they work in a weird way. Most mailers take a single user ID (the recipient), look up their notification email, and go from there. Pipelines just grab the user's global notification email (https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.10.0/app/services/notification_service.rb#L419) and pass that to the mailer as a single BCCed list (https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.10.0/app/mailers/emails/pipelines.rb#L21-29).

This should not be necessary: other emails can have lots of recipients too, and they work fine. If we change the pipeline emails to use recipients like other emails, we can look up the correct email using the existing method (https://gitlab.com/gitlab-org/gitlab-ce/blob/v11.10.0/app/mailers/emails/pipelines.rb#L21-29).