[FIX] mail: Discuss: don't suggest to join private channels
[FIX] mail: Discuss: don't suggest to join private channels

Problem was that administrators have access to every channels, even private
channels they are not registered to, because when trying to join an existing
channel, the autocomplete proposed every channels accessible for the user, but
didn't filter private ones.

This rev. makes sure to only suggest non-private channels.

Also prevent '|' domain injection to disable filters (odoo/odoo@44e65e72d3a362dedf2e7084ae193b5d8d4be3ad)
by using expression.py to build domain.

laslabs/odoo,microcom/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,laslabs/odoo,ygol/odoo,stephen144/odoo,stephen144/odoo,dfang/odoo,microcom/odoo,ygol/odoo,storm-computers/odoo,dfang/odoo,laslabs/odoo,stephen144/odoo,sysadminmatmoz/OCB,stephen144/odoo,bplancher/odoo,microcom/odoo,storm-computers/odoo,storm-computers/odoo,bplancher/odoo,dfang/odoo,sysadminmatmoz/OCB,bplancher/odoo,hip-odoo/odoo,laslabs/odoo,bplancher/odoo,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,dfang/odoo,sysadminmatmoz/OCB,ygol/odoo,sysadminmatmoz/OCB,dfang/odoo,Elico-Corp/odoo_OCB,microcom/odoo,dfang/odoo,storm-computers/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,microcom/odoo,hip-odoo/odoo,storm-computers/odoo,ygol/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,microcom/odoo,sysadminmatmoz/OCB,ygol/odoo,hip-odoo/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,stephen144/odoo,ygol/odoo,ygol/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,hip-odoo/odoo,stephen144/odoo,bplancher/odoo