Create email alias service for BH users

[ahf]

We have different situations where it would be beneficial that different teams can list different subsets of users in our system, but we shouldn't expose people's actual email addresses to volunteers.

It would be useful to have, as part of our web application, that every BH user have a read-only email "account" such as ${username}@users.bornhack.org which we can expose to volunteers where users are reachable over ordinary email.

People should probably have an opt-in/opt-out mechanism for disabling this and the "shadow" email should thus be unavailable.