Original PR description

Purpose
=======

For objects inherting from rating.mixin, when their parent model change
(e.i.: task from a project to another), the "Parent document name" does
not change. The rating is still considered belonging to the old parent
object (rating stat of project is not correct).

Specification
=============

Change the rating API. In rating mixin, instead of having 2 methods
returning parent id and parent model (rating_get_parent_id and
rating_get_parent_model_name), we should have one method returning the
'parent relation field' (inevitably m2o field). Then we can deduce its
parent_res_id/parent_res_model, and check this parent field is in values
of write to trigger the recompute of parent_res_name.

Also, add "ondelete=cascade" on parent_res_model_id

Impacted modules: project, rating, helpdesk, livechat