[IMP] website_forum: make forum;forum inherit from mail.thread. This way we

[IMP] website_forum: make forum;forum inherit from mail.thread. This way we
add the possibility to users to follow a forum and be automatically followers of new
questions, using some new subtypes on the forum.forum model that do the auto subscription.

Also added a profile link on the forum, when logged.

Also added a subscribe button on the forum.