Add magic-link login option

[tg44]

The use-case:

• User "registers" with an email address.
• User gets a "registration" email, clicks on the link
• The link creates the new account
• When the users try to log in again, they can write in the mail address to the login form (only email)
• User gets an email with "magic link" in it
• The link logs the user in

This probably needs a new email template (if we have one), it needs a new amqp topic too.

Probably the whole flow can be added to the already existing email-pass login flow.