@-Mention Slack integration

Release notes

With this change we expand notification events in GitLab for Slack app. Now you can trigger notifications for group mentions in issue and merge request descriptions, comments on issues, merge requests, and commits.

Description

An integration with Slack that creates a message in a particular channel for @-mentions of particular teams (ex gitlab-com/gl-security/appsec). Some additional information and history can be found in this previous issue describing this need for AppSec.

Why is this needed?

• Increases visibility for pings
• Makes it easier to collaborate/coordinate on who is going to handle these pings
• Allows us to use a ✅ or other indicator to make it clear that it has been reviewed
• (for AppSec) it's a core part of our triage rotation

What are the resource savings (time and cost)?

• Reduces team member time overlap in ping responses
• Reduces time other teams spend being blocked by or waiting for a response
• (for AppSec) Our current implementation is prone to breaking and is only accessible/fixable by a Senior Manager

Which team is requesting this?

AppSec specifically, although other teams would certainly benefit from it. @agroleau mentioned that this might even be a feature for the product itself.

What is your definition of done?

@-mentions for our group showing up in the #sec-appsec Slack channel