Fix for two column matcher shuffling both columns identically.

Summary:
The same seed was being used to create an RNG for both columns, which by
definition meant that distinct sequences of equal length would be
shuffled in the same way. This fix creates the RNG once, then reuses it.

Test Plan:
Created a matcher item with six entries on each side in test.html,
observed that the two columns were shuffled independently.

Reviewers: jack

Reviewed By: jack

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D6482