STUD-669.

We used to use a regular JS `confirm`; this makes the UI consistent
with the rest of Studio.

Also change the file uploader to accept multiple file types.

- Internationalize upload errors.
- Move upload tests into their own files.
- Refactor upload dialog acceptance tests.

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.

isn't supported in Firefox.

Clicking "Sync to <course>" should send an AJAX request with the datetimes
set to null, so that the server resets them.

Firefox wasn't saving section release times, due to issues with JS Date() parsing.
I've modified the code to make it more explicit around what it should do and how
it should work, which also makes it work better with both Firefox and Chrome.

Clicking "Sync to <course>" should send an AJAX request with the datetimes
set to null, so that the server resets them.