Email specific users in escalation policy

For the MVC, people will only be able to escalate to an already-created schedule (see proposal here). We should follow this MVC with the additional option to escalate to a specific user.

Release notes

When creating the escalation policy for alerts, it is useful to be able to email a specific user. In this release, we added the ability for teams to define exactly whom to contact, even if that specific user is not part of the on-call rotation.