Add unique email constraint on user model

[otaviojacobi]

This enforces database level uniqueness on the email field. I opted for keeping the current hooks alongside this (see https://github.com/balena-io/open-balena-api/blob/d3b61faad9d3aeb6fa22f129e70c2a75ce2f3070/src/features/auth/hooks/validate-username-email.ts#L45) in order to keep the error messages clean. In the case of a PATCH for a user with an email that is duplicated, without this hook api would respond with 200 and just don't do anything, so I decided to keep it. As the case where the hook fail is a very rare race condition I could not figure it out how to test the constraint.

Change-type: minor

[joshbwlng]

I'd guess we'd want to reduce hook overhead and let the database error on duplicates at some point in the future, but I get the point of not wanting to do this now. If I understand correctly that would probably result in a major and not just a minor (response code changes, error message changes). Dropping the hook check would allow for testing the database-level constraint later on.