Implement Track Changes

@stephensanchez please. I'll be on vacation next week, but @caesar2164 and @stvstnfrd will be around and can handle our end of the PR process.

* 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>
This is an IIFE [1]. It leverages JavaScript's function-based scoping to
provide an isolated local scope. This ensures that no local variable
declarations are implicitly added to the global namespace. It also
allows us to enable strict mode for the entire file/module without
applying these strict requirements to nearby code.

In browser applications, the `window` object represents the global
namespace; it is the default value for `this`. By passing the `window`
object as a parameter to the IIFE, the code is more modular and explicit
by ensuring that accessing any global property is done explicitly
through the `window` object. This behavior is enforced with a linter.

Dereferencing properties from the global object makes code:
- more performant by preventing namespace resolution from traversing the
  scope tree upon access of each property
- more obvious by removing implicit globals properties

[1] http://en.wikipedia.org/wiki/Immediately-invoked_function_expression