saloon: improve user-registration process with email-confirmation

[sbinet]

we should make sure the email associated with a username is:

• a valid one
• controlled by the user on the other end to prevent unwanted spam.

therefore, we should create new users in a "blocked", "unactivated" state until a secret link sent to the user by email is clicked (or some secret passphrase is entered in a dedicated form.)

we should make sure we don't send emails to unactivated users.