Pagination should be standard and easy to adopt so moved the paginators
into the edx_rest_framework_extensions package so they can be easily
installed into other app/projects
* Catalog results are now paginated
* Implements the new namespaced pagination described at
https://openedx.atlassian.net/wiki/pages/viewpage.action?pageId=47481813
* API level code returns pythonic business objects
* View layer performs serialize at the view layer
* Convert views to use DRF generic views
* Removes an unintentional authentication decorator that caused
the detail endpoint to return a 401 for anonymous users
MA-1724
