Send emails when email address is added/changed

One to us, one to them; contextually aware of whether email address is
being set or updated; if it's being set and the user has RSVPed, we
include their RSVP status as they won't have been emailed about that; if
it's being updated, they've already been emailed their RSVP status
either when they RSVPed or when they set their email address.
Merge branch 'feature/email-address-email' into develop