Limit automation rule form `alertRecipients` field to existing users

[nathanielrindlaub]

Currently we allow users to add a comma-separated list of any email addresses, but we should limit this to existing users within that project.

[lessej]

I think it's worth considering email distribution lists here. I could see a scenario where project teams want a single email, like [email protected], to be notified.

I can also see the merit of allowing arbitrary email addresses for integrations / automations with other platforms. For example, in CS we have a Workday -> Jira integration where Workday sends alarms to a Jira email address that automatically creates tickets.

A best-of-both-worlds solution might be adding a dropdown that also allows arbitrary input if nothing matches