moved setting of assementMode into prepareSite which is called after Exercises.init()

Summary: assessmentMode wasn't being set correctly because Exercise.init() had not been called yet

Test Plan: Load /labs/assessments/fractions page and get a problem wrong and make sure that it doesn't say try again

Reviewers: chris, jace

CC: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D1074