Add mobile support for the unit widget.

Summary: Everything in here is pretty straighforward, I believe.

Test Plan:
* `make test` passed, but I don't think it tested any of this.
* I plan to test this on mobile as soon as it's shipped.

Reviewers: emily, aria

Reviewed By: aria

Differential Revision: https://phabricator.khanacademy.org/D15362