Consolidate access checks for prereqs and surveys.

common/djangoapps/student/views.py

lms/djangoapps/courseware/access.py

lms/djangoapps/courseware/access_response.py

lms/djangoapps/courseware/courses.py

lms/djangoapps/courseware/exceptions.py

lms/djangoapps/courseware/tests/test_access.py

lms/djangoapps/courseware/views/index.py

lms/djangoapps/courseware/views/views.py

lms/djangoapps/mobile_api/decorators.py

lms/djangoapps/mobile_api/tests/test_milestones.py

lms/djangoapps/mobile_api/users/tests.py

lms/djangoapps/survey/tests/test_utils.py

lms/djangoapps/survey/utils.py