Preview: Track Changes initial implementation

@caesar2164 @stvstnfrd @jrbl @jbau Let's do some pre-review review. This is a PR against origin/master, NOT upstream/master.

* Creates "TrackChanges" django model, which ties a (submission_uuid, scorer_id) pair to a set of edits suggested by peers for a particular submission.
* Adds machinery for on-the-fly installation of the New York Times's ICE library for collaborative editing, allowing peer assessors to create marked-up versions of submissions, which can then be returned to the author as feedback.
* Adds machinery for displaying suggested edits to the creator of a submission with their scores and other feedback.
* Configurable within Studio on a per-Assessment basis by including a URL to the ICE library as part of the assessment definition.
 * For additional details, see:
   https://docs.google.com/document/d/18PITJu133sFpDl_B2GCbo2eb0GODcwVmVsktwPQfVSg/edit?usp=sharing
    
Co-authored-by: Giulio Gratta <giulio@giuliogratta.com>
Co-authored-by: Steven Burch <stv@stanford.edu>
Co-authored-by: Joe Blaylock <jrbl@jrbl.org>
Unused import.