Password reset and account email

Accounts (#126) have a username and a password only: a tester who forgets the password is locked out until an operator helps. Add an email to the account (optional at registration), a "forgot password" flow (a one-time link or code by email, through the same mail service as the waitlist), changing the password and the email from the character screen, and an admin way to reset a password by hand meanwhile.

Done when a tester can get back into a forgotten account without an operator.