[REM] mail: remove o_mail_note class in chatter

Purpose of this commit is to clean a bit various classes and conditions.

First clean is the removal of o_mail_note class in activities less. It comes
from first implementation of activity widget and is not used anymore in
activity widgets rendering.

Second clean is the removal of o_mail_note class when displaying messages.
Only o_mail_discussion is used currently in chatter to make a different
background depending on whether the message is a discussion or not. All
we have to know to differentiate message display is whether it is a
discussion done by an user or not.

Tests have been updated to use o_mail_discussion class triggered by
the message being a discussion, aka a comment posted by an user.