* 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
This is the first in a series of commits to create
a Python API for the LMS program_enrollments app.
We do some general refactoring, renaming, and clean-up
in order to move toward the creation of that API.
EDUCATOR-4321
