Master rating jem

* Task : https://www.odoo.com/web#id=8060&view_type=form&model=project.task&menu_id=3942&action=327
* Pad : https://pad.odoo.com/p/r.2BmKqyvcwKbiXPRI

Rating modules 
* rating : with the rating mixin and rating objects
* rating_project : apply rating for task project, by email
* rating_project_issue : idem, but for issue
* rating_livechat : rating for the livechat, using the mixin
* website_livechat : modified to include web pages exposing the rating of the channels