Generalize file uploader.

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.
Allow course image uploads in the settings page.

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.
Address PR comments:

- Internationalize upload errors.
- Move upload tests into their own files.
- Refactor upload dialog acceptance tests.
Studio: revises styling/copy around course image management in settings
Add support for PNGs as course images.

Also change the file uploader to accept multiple file types.
Update CHANGELOG.
Update copy to include PNGs, add tests, remove default MIME type.