Expression: Move expression above scratchpad

Summary:
We were't applying the ApiClassNames.INTERACTIVE class to expressions,
so you could draw over expressions with the scratchpad. This could
lead to some unfortunate situations where users would draw in the
expression box (or otherwise be unable to answer the question :P).

This commit also slaps some INTERACTIVE classnames on
InputWithExamples, because anywhere you use that you really should
be using the .INTERACTIVE classname anyways.

Test Plan:
 * subperseus
 * open "writing expressions with exponents"
 * turn on the scratchpad
 * click on the expression and be able to type into it.

miniondance

Auditors: charlie