Email validation

[johanvdw]

cfr pretalx/pretalx#846
right - well that issue is pretty worrying to me - it was opened in 2019 yet is very basic stuff
it's one of those things we'll end up having to fix ourselves...

that proposal is too complicated

it only needs something simple like pentabarf did - a single flag on each email address indicating that it has been verified, and the email generating system ignorining everything that doesn't have the flag

plus a trigger to send the verification email, which I think the system already does

ie a very simple bit of code that guarantees we never send emails to unverified addresses - apart from the email that verifies them