Add more to distinguish between different alerts in Slack

### Problem Statement

Sentry's Slack alerts make it hard to distinguish between different kinds of alerts.

In this example, there are two alerts, one that has happened many times, and one that has happened a single time.

I've set up two different Sentry alerts: "High frequency error - app", and "New issue - app", but the only thing that distinguishes them is the subtitle at the bottom of the alert.

It is very easy to miss this and for your eyes to glaze over the differences. In this case, it meant that it took us longer to spot this alert than it should have.

### Solution Brainstorm

It would be great if we could customise the emoji used in the Slack alert, and maybe add a prefix to the error message:

### Product Area

Alerts