Hide show_timezone from Advanced Settings and migrate courses with show_timezone=False to using due_date_display_format.

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.

Instead, we use XModule field default values when creating an empty
XModule. Driven by this use case, we also allow for XModules to be
created in memory without being persisted to the database at all. This
necessitates a change to the Modulestore api, replacing clone_item with
create_draft and save_xmodule.

Make default value for format setting None, and force lms and cms to be explicit about a string default

don't make caller responsible.

definition not being updated when using defaults. Not sure why this
passed before.