Update styles of old expression edtior

Summary: The old expression editor is still shown on Windows/Chrome and all touch devices (e.g. iOS, Android phones, etc) and is clunky to work with because the tooltip explaining commands appears over top of the output. I've moved the output above the input, and added some styles to the input + output.

Test Plan: Play with the old-expression editor and judge the new styles.

Reviewers: eater, aria

Reviewed By: aria

Subscribers: alex, marcos

Maniphest Tasks: T2936

Differential Revision: http://phabricator.khanacademy.org/D12548