Add attributes to submission and final submission file

- add fedora_id to submissions
- add content_type to final_submission_files
- in preparation for Fedora depositing
Rename StudentPaper to Paper
Remove mock depositor

Not clear whether we need this.
Add a basic ScholarSphereDepositor class

Still need to add final submission files to the
Paper (ActiveFedora) model.
WIP: Assign content_type to final_submission_file