Add buttons to expression widget.

Summary:
Things to ponder:

* Mocks from Elizabeth:
    - https://s3.amazonaws.com/uploads.hipchat.com/6574/805139/jQzWv3OdQOQm8MN/upload.png
    - https://s3.amazonaws.com/uploads.hipchat.com/6574/805139/74kmSqXTfYyOWXb/1-Expressions.jpg
* The mocks are a very loose approximation to what will be built in this
  iteration. Notable differences:
    - No calculator button - you click into the input to open the
      "calculator" buttons.
    - Different button sets. We're providing four preconfigured sets of
      buttons for question authors to choose from so we don't have the
      luxury of a nice 3x3 rectangle of buttons.
    - I suspect our final result will have significantly larger buttons
      due to readability / size issues.
    - No examples button for now.
    - I might or might not cut the iterated "don't understand that"
      communication for this round.
    - I still have a some design work left to do to figure out how to make any
      of the possible sets of buttons look good. Right now I cap with width at
      200px or something and let the buttons wrap. I included a white
      translucent backdrop to make it less noisy / more readable over black
      text, but that looks bad when there are corners with no buttons.
* Our chosen symbol sets are mostly drawn from here
  http://practice.parcc.testnav.com/resources/HS_EE_Tips_v0.1.pdf
* Desmos can handle \sqrt[3]{x} but I haven't yet investigated how.
  Vanilla mathquill can't.
* It's a bit awkward for buttonSet to be a property of MathInput but
  configured on Expression, so you have to pass it through
  InputWithExamples.

Test Plan: TODO - for now just playing with it on localhost:9000/test.html

Reviewers: jack

Reviewed By: jack

Subscribers: elizabethlin, alex, eater

Differential Revision: http://phabricator.khanacademy.org/D10012