[IMP] mail: allow user to delete their messages

This commit allows users to "delete" their messages in discuss, as well
as their notes on other records. Admins can delete anyone discuss
messages or notes. Tracking messages such as a change in stage or other
cannot be deleted.

This "delete" is a "soft-delete" in that it only empties the message's
content, so that it is no longer displayed in the UI, but remains in the
database so that people who got notified by email can still reply to the
message, among other things.

task-2365646

closes odoo/odoo#74857

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>