Authors can upload an image (or choose an existing one) from the
settings page, using the in-context uploader from PDF
textbooks. Includes tests for backwards compatibility with XML courses
-- they used a magic filename (images/course_image.jpg) which is
mapped to a location in the Mongo contentstore.

Still needs some UX work, though the backend plumbing is there.

Previously the file upload dialog was PDF- and textbook-specific. The
changes are adding parameters to the FileUpload model for the file
type, and adding an onSuccess callback to the UploadDialog view. Also
moved upload-specific SASS into its own file.

put a 4th field in the create new course form where the user must specify a course_run. Also update unit tests to also provide this 4th parameter. Styling changes to better lay out the form fields are still TBD.

Conflicts:
	cms/templates/howitworks.html

Studio: updates older extend references and solves Sass warnings/dupe display of accessibility-based text