Alerts by occurrences for the same user

### Problem Statement

We'd like to set up alerts when there's a # of issues over a certain time period, but also for a particular user. That is, if `User A ` experiences `Error 1` at a rate of 3 occurrences / 5 minutes, then Sentry triggers an alert. If `User A`, `User B`, and `User C` experiences `Error 1` at a rate of 1 occurrence _each_ in a span of 5 minutes, then we do _not_ trigger an alert.

Our goal is to proactively reach out to users if they are having trouble using our app, and so that is the motivation behind the # occurrences / time span for a particular user

### Solution Brainstorm

Add an additional WHEN or IF condition on the Sentry Alert `Edit Alert Rule` page to specify "for the same user".

### Product Area

Alerts