This commit updates common/djangoapps.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

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