[FIX] mail: messages sent as reply to previously selected message
[FIX] mail: messages sent as reply to previously selected message

In Discuss, if a message was selected as message to reply to, it wasn't reset
when switching to another channel. So if the user sent a message in that other
channel, the message was sent as reply to the previously selected message.

This rev. ensures to always unselect messages when switching between channels.

Closes #11879

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