Don't crash on invalid invoice recipient email address

If a username was *not* specified, but something was entered in
the email field that was not a valid email address, the form would
crash. Instead, we should just show the message that it's an
invalid email address, of course..
Make it possible to set a "last available date" for registration types

This is typically used for Early Bird or similar campaign rates
Show regtype activeuntil in the admin listing