Create `<ActiveZone>`.

Summary:
I can't overstate how helpful this has been to clarify the semantics of
the expression buttons and simplify in general.

Test Plan:
* Opened an expression question in dev:9000/test.html
* clicked in the expression input, gained focus
* clicked everywhere on the buttons popup, focus remained
* clicked outside it, lost focus
* typed inside, focus remained

Reviewers: jack

Differential Revision: http://phabricator.khanacademy.org/D10410