Support alerting once an issue is assigned to a person/team

### Problem Statement I want to be alerted once when an Issue is assigned to me. As of right now, Sentry does not support this use case since a new event will fire the alert which will result in redundant alerting. ### Solution Brainstorm Having the option to be notified once for an issue in the UI would be great. The current workaround is setting a high action interval.