[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
Create a genereic method get_stat_ratings on the mixin, using a read_group (for example : read_group([('res_model', '=', res_model), ('res_id', 'in', res_ids)], ['state', 'id'], ['state', 'res_id']) ) to extract for each res_id a dict containing for each possible state the number of rating of this state.
Exemple : {'id' : {
'bad' : 5,
'ok' : 7,
'great' : 12,
'no_vote' : 2
}
}

Then use get_stat_ratings, to comput the number of rated tasks, happy task, rated issues, ...