Make too-wide questions overflow

Summary:
Makes the question/hint width a fixed 480px, and sets content
to overflow rather than resize that width. If a question writer
creates a table that is too wide, it will be obvious that it
is too wide.

Note: this behaviour only happens in the editor, so rendered
questions will appear the same, and will take advantage of
students' screen real-estate, if available.

Test Plan:
Add the following text to the question, answer, and hint in
test.html:

header 1 | header 2 | header 3
- | - | -
data 1 | data 2 | data 3 | a | bafdasfafdasf | b| bdsafasfas| b| b|
b|adsffadd b| b| b| b| b|

Reviewers: brianmerlob

Reviewed By: brianmerlob

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D6940