initial commit of sneak peek support for courses

* some more tests

* make preview work with shib classes

* Styled unauth access button and banner.
  - renamed "preview" button to "Explore Course" and styled.
  - moved banner from main navigation to course navigation.
  - moved span tag inside link to make clickable area larger

Conflicts:
	common/djangoapps/student/tests/factories.py
	common/djangoapps/student/views.py
	lms/djangoapps/courseware/tests/test_access.py
adding unauthenticated use footer
Jeffs 3x merged into one

Conflicts:
	CHANGELOG.rst