STUD-733

due to the MixedModuleStore pending release, we need to make enrollment start/end dates editable in Studio

STUD-669.

  Comments, httpresponse -> jsonresponse; make sure errors get relayed.

- 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.

This was confusing translators as didn't make sense in English

switch the notification to be a prompt and allow for the user to go to the edit unit page which contains the module in error. Otherwise, present the raw exception message and allow user to go to the course outline page.

Also fixes a bug where message was not sent to user when entering 'denied'
state unless the user was previously in 'granted'.

Send notification to admin when a user enters 'pending' state.

Pylint cleanup.