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).

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