Allowing file based submissions for ORA2

Allows upload via fileupload service, implemented to use s3
UI is pretty basic for now. Allows upload, viewing images from the same service.
@wedaly 

If this becomes a service outside of ORA2, it might be worth making another setting for this that's specific to this service.  We can always assign it to `ORA2_FILE_STORAGE_PREFIX` in the edx-platform settings if we want to store things in the same bucket.