[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
Using a generic method on the mixin that returns for each given id, the number of rate groupy by 'state' will replace '_get_total_happy_task' and '_get_total_happy_issue' method (which are doing the same thing on different model), so we will reduce the code to maintain. 
 I don't understand why you need only the last rating ? But if you need it, you can also create a generic method on the mixin (it could be useful for other cases).
 For the selection list of the 'state' field, you are right for the zero and False value in database. I suggest [(-1, 'No Vote'), (0, 'Bad'), (5, 'Ok'), (10, 'Great')]. This will allow a better reporting for graph view (with sum, avg, ...). Also, put the -1 value as the default.