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

[MERGE] 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.

Misc :
- also added a profile link on the forum, when logged.
- also added a subscribe button on the forum.