Replace all clean_course_id form methods with common method.

cms/djangoapps/contentstore/config/forms.py

cms/djangoapps/xblock_config/forms.py

common/djangoapps/course_modes/admin.py

common/djangoapps/student/admin.py

common/djangoapps/student/tests/test_admin_views.py

lms/djangoapps/bulk_email/forms.py

lms/djangoapps/bulk_email/tests/test_forms.py

lms/djangoapps/grades/config/forms.py

lms/djangoapps/support/tests/test_refund.py

lms/djangoapps/support/views/refund.py

openedx/core/djangoapps/video_config/forms.py

openedx/core/lib/courses.py