Two-column styles for SAT

Test Plan:
Load up /mission/sat and do a reading section.

Reviewers: aria, johnsullivan
Use LESS var for SAT height
Two-column styles for SAT

Summary:
Use LESS var for SAT height

Mods to make two-column SAT layout work

Test Plan:
Load up /mission/sat and do a reading section.

Go to /mission/sat, fire up reading section, check out single- and two-column views

Reviewers: aria, johnsullivan

Subscribers: dylan

Differential Revision: http://phabricator.khanacademy.org/D14271