[FIX] mail: various fixes

- mail_message: fixed internal subtypes

Indeed has_group works only for uid and does not care for any given recordset.
Probably we need a is_in_group method on res.users. Someone smart could code it.

- mail_message: update message before notify

This way in the various notify methods, the message is correclty updated with
needaction and recipients. Also cleaned some comments.

- mail_message: avoid email loops

Do not notify people having the same email as the message / email to propagate.

- mail_message: fix access right issues

When computing recipients and partners to email, use a sudo for some internal
computation to avoid access rights issues. Indeed public user may go through
this method (forum for example).