Mention assignee on slack alerts

### Problem Statement

The alerts send on a channel gets no attention till someone mentions who it is assigned to, since the assignee is only tagged at that point. Why can't sentry tag the assignee along with the message in both the initial message and the follow-up message which it sends in the thread.
This could be a step forward in fully automation the escalation and alert system using sentry.

### Solution Brainstorm

- permanently tag the assignee if he/she is available present in slack when sending an alert, you can use email look-up.

### Product Area

Alerts