Restrict notifications for subscribers of the confidential issue's labels

For confidential issues:

• When is created, notify only the subscribers of all its labels that can read the issue.
• When is edited, notify only the subscribers of all its newly-added labels that can read the issue.

Reassigned to @dbalexandre

mentioned in merge request !3334 (merged)

Added regression label

Status changed to closed by commit 3b39ce32

mentioned in commit 3b39ce32

mentioned in commit 3536b17e