Merge pull request #6784 from edx/will/invoice-refactor-squashed

common/djangoapps/student/tests/tests.py

common/djangoapps/student/views.py

lms/djangoapps/instructor/tests/test_api.py

lms/djangoapps/instructor/views/api.py

lms/djangoapps/instructor_analytics/basic.py

lms/djangoapps/instructor_analytics/tests/test_basic.py

lms/djangoapps/shoppingcart/admin.py

...pps/shoppingcart/migrations/0025_update_invoice_models.py

...angoapps/shoppingcart/migrations/0026_migrate_invoices.py

lms/djangoapps/shoppingcart/models.py

lms/djangoapps/shoppingcart/tests/test_views.py

lms/static/sass/course/instructor/_instructor_2.scss

...mplates/instructor/instructor_dashboard_2/e-commerce.html

...tructor_dashboard_2/generate_registarion_codes_modal.html