[IMP] website_mail: refactor follow button; add partner_id in session for public user
[FIX] [IMP] website_mail: improved and fixed the 'subscribe button'.
This button is used to perform a 'message_subscribe' or 'message_unsubscribe' on some
documents. It is notably used to add a 'Subscribe' button in some pages that subscribe
the user to a mailing list (aka mail.group).
This branch fixes some subscribe/unsubscribe issues, better manages the public user,
use the session to remember whether a public user has subscribe to a mailing list,
and uses already-existing code to manage partners.