[IMP] portal: add informative button to internal notes on portal chatter

PURPOSE

Before this commit, when the chatter was displayed on a portal page (with
the customer preview button) one could see the internal notes with a button
for toggling his visibility... which was ineffective because a portal user
could never see internal notes and on the opposite internal users could
always see them. This commit adds another informative (disabled) button for
internal notes.

SPECIFICATION

Adds a 'is_message_subtype_note' param on the message in the
_portal_message_format method.

LINKS

Task-2817892

closes odoo/odoo#88310

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>