Verification: abstracting out specific color-based Sass variables to be *somewhat* friendly to theming

edx.org: revises course register button background for all cases (verified, riught column, and bottom of page)

edx.org: adds background to register button's html body element for visual blending in course detail view

Conflicts:
	common/djangoapps/course_modes/models.py
	lms/djangoapps/shoppingcart/models.py
	lms/djangoapps/shoppingcart/processors/CyberSource.py
	lms/djangoapps/shoppingcart/processors/tests/test_CyberSource.py
	lms/djangoapps/shoppingcart/tests/test_models.py
	lms/djangoapps/shoppingcart/tests/test_views.py
	lms/djangoapps/shoppingcart/urls.py
	lms/djangoapps/shoppingcart/views.py
	lms/envs/common.py
	lms/envs/dev.py
	lms/static/sass/base/_variables.scss

Mark tests for unimplemented features as skip so that the acceptance tests will all pass when we merge into master.

Verification: syncing all submit button-type styling (including newly used element on verified view)