[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
You should create a method on the Mixin, call get_stat(self, res_model, res_ids). This method will return a dict such as 
{'id' : {
            'bad' : 5,
            'ok' : 7,
            'great' : 12,
            'no_vote' : 2
        }
}

Then, use this method to compute percent_happy_task and happy_customer by summign the values. Then, you should have less method to compute your 2 fields.