Adding a label to an epic does not generate a notification

Summary

When a user is subscribed to a group label and the label is added to an epic the user should receive an email notification but no notification is generated.

Steps to reproduce

1. Subsribe to a group label
2. Add a label to an epic

Example Project

I used this epic: &304 (closed) and added create label. No notification was generated for a user who was subscribed to the label. I confirmed labels notifications are working on this issue: jarka/public-test#1

What is the current bug behavior?

Users (who are subscribed to the label) don't receive any notification when a label was added.

What is the expected correct behavior?

Users receive a notification when a label they're subscribed to was added.

Output of checks

This bug happens on GitLab.com

/cc @victorwu