Add option for disabling email notifications for confidential issues

In The Tor Project, we have just moved from Trac to Gitlab CE. We are very happy to finally have a "Confidential Issues" feature, that allows some secrecy between the issue reporter and the people in the project trying to handle a potential security issue.

One feature that would be useful for us to have, would be if it would be possible to disable email notifications on confidential issues. Our developers comes from all around the world and uses different email providers, and we would prefer if we wouldn't have to trust any of these.

One option for us is to handle this on the outgoing email server that our Gitlab instance uses, but if an option is possible in Gitlab itself, that would be neat.

Proposal

• Add a setting via the cascading settings framework to optionally disable sending notifications for confidential issues.
• The setting should be defaulted to off at the instance level.
• The setting should be defaulted to unlocked
• The setting is available at the instance, group, and project levels