[FIX] mail, website_mail: specific mailing-list footer + better unsubscribe option

- remove the default footer for mail.group messages,
  replace with specific footer with archive and unsubscribe
  link
- remove the automatic addition of user signature in
  mail.group messages, as many of them will be posted
  via the mail gateway and already contain a user signature.
- make it easier to unsubscribe even when not logged in,
  as followers who have not signed up will have no
  way to login short of signing up.
- remove tests looking for user signature in mail.group posts