Paginate course card API for Programs Learner Portal

* Pull business logic of ProgramCourseEnrollmentOverviewView
  out of view class and into utils.py.
* Add UserProgramCourseEnrollmentsView, which is a paginated
  version of ProgramCourseEnrollmentOverviewView with a
  URL that is parameterized on the user (to enable masquerading
  in MST-109).
* Add get_certificates_for_user_by_course_keys to certs API
  to make enrollments overviews REST API use fewer SQL queries.
* Document new course cards API with edx-api-doc-tools.

In a follow-up ticket, the Programs Learner Portal will switch
to the new paginatd API in order to speed up its page load.

MST-126