users: Disallow new users with existing verified email

Created by: ryanslade

We should not allow new users to sign up with an email address that is already verified and assigned to another user.

We chose not to add a new constraint as it would require a migration and it's not obvious what to do with existing accounts that would now be considered invalid. It's simpler to just not allow this moving forward.

Fixes: https://github.com/sourcegraph/sourcegraph/issues/22447