Add ability to set selected users for Slack notifications from Aha!

What is the challenge?

Currently, the newest Aha! Slack integration does not allow you to set specific users to trigger a notification to Slack from Aha! in the way the previous version supported.

What is the impact?

You cannot filter down activity by user which makes it challenging to route slack notifications to specific channels with relevant users.

Describe your idea

Enable the previous capability to limit which users trigger the notification.