[ADD] added rating, rating_project and rating_project_issue module

 - Task : https://www.odoo.com/web#id=8060&view_type=form&model=project.task
 - Pad : https://pad.odoo.com/p/r.2BmKqyvcwKbiXPRI

[IMP] Added option to install rating module under Project

[IMP] added template to display when customer rates the task/issue

[IMP] Improved template and code

[IMP] : rating , rating_project and rating_project_issue as per new api

[IMP]: rating template

[FIX] : test case failed

[IMP] : code
Actually, in the case of rating_project and rating_project_issue, you must keep only the last rating. So when a task stage has changed, you must not direclty create a new rating : re-use on old one if exists, and update its access_token, and state. 
Here call a method get_rating(values) which will search the record for the given res_model and res_id. If don't exists, create one, else update it.