Improve initial experience for invited users

Currently, invited users have to copy a password from an email to log in first. This PR adds code that allows administrators (in a future change to massmailer) to invite users with a short *welcome* URL, looking like

    http://adhocracy.example/welcome/Jane_Smith/f1a2c3e4

Upon clicking that link, the user is signed in and is prominently asked to pick her own password.

Current installations should not be affected since the option `adhocracy.enable_welcome` defaults to False.