Teach pgweb to handle secondary email addresses

Teach pgweb to handle secondary email addresses

This allows each account to have more than one email address, of which
one is primary. Adding more addresses will trigger an email with a
verification link (of course). The field previously known as "email" is
now changed to be "primary email".

Change the profile form to allow freely changing between the added
addresses which one is the primary. Remove the functionality to directly
change the primary email -- instead one has to add a new address first
and then change to that one, which simplifies several things in the
handling.