Django-courseaffils

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* Added Affil.shortname

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* Added Affil.past_present_future

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* Introduced the Affil model for storing affils from CAS that don't yet
have an associated Course.
* Changed CourseManagerMiddleware so faculty always see the switch course
view, and are never automatically forwarded into a course.

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* Allow CourseListView to be overridden in CourseManagerMiddleware.

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* optimize CourseListView rendering

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* columbia.CourseStringMapper now has to_dict and to_string methods.