Defer local mode chrome loading until asked

Summary: I kind of hate this but I haven't found anything better that doesn't require reorganizing basically everything.

Test Plan: Loaded /exercises/addition_1.html, saw a problem. Commented out the loadLocalModeSiteWhenReady call in local-only/main.js, reloaded, and didn't see a problem.

Reviewers: jack, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D7232