Added an expiration_datetime field to CourseModes, intended to eventually replace expiration_date.
Included relevant schema and data migration file.
LMS-1454
With tests, some settings changes
(all should default to not breaking anything for edx)
Added styling for shopping cart User Experience
- Styled shoppingcart list page
- Styled navigation shopping cart button
- Styled receipt page
- Styled course about page for shopping cart courses
Addressed HTML/SCSS issues
Remove offending body class and unnecessary sass changes
Addresses many review comments on stanford shopping cart
* framework for generating order instructions on receipt page
in shoppingcart.models
* move user_cart_has_item into shoppingcart.models
* move min_course_price_for_currency into course_modes.models
* remove auto activation on purchase
* 2-space indents in templates
* etc
revert indentation on navigation.html for ease of review
pep8 pylint
move logging/error handling from shoppingcart view to model
Addressing @dave changes