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.

Update existing tests and provide new ones to test new paradigm.

a partial descriptor. 
Use 400 not the contrived 515 for malformed html.

Addressed some of Christina's review comments.

speeds and don't use the video/default.yaml format.

Add unit tests.

change.