[IMP] im_livechat: make channel use rating parent mixin
[IMP] im_livechat: make channel use rating parent mixin

No functionnal changes are done in this commit. Even
the period of rating to take into account to compute
livechat channel satisfaction (introduced at
5b60e05ce4ed0531a219517855247757841786af)
stays the same, but use the parent mixin mecanism.

Task-1903565
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo