Refactor Courseware Index

common/djangoapps/student/tests/tests.py

lms/djangoapps/courseware/entrance_exams.py

lms/djangoapps/courseware/exceptions.py

lms/djangoapps/courseware/index.py

lms/djangoapps/courseware/module_render.py

lms/djangoapps/courseware/tests/test_entrance_exam.py

lms/djangoapps/courseware/tests/test_module_render.py

lms/djangoapps/courseware/tests/test_navigation.py

lms/djangoapps/courseware/tests/test_views.py

lms/djangoapps/courseware/views.py

lms/djangoapps/mobile_api/users/views.py

lms/urls.py