[bug 956836] Replace tastypie with django-rest-framework.

KPI app was using tastypie. Now it uses DRF.

I did the minimal thing possible to replace the tastypie views with DRF views. Therefore, there are likely lots of improvements that can be done to better use DRF. But the goal was to get rid of tastypie ASAP without breaking anything, so that's what I did for now.

I just browsed through the diff and github did a great job. I thought the diff would be a mess.

r?