Add Optimizely to all pages. Bug 1073115.

I believe these changes will add the Optimizely JS to all pages (before the Google Analytics script). First time contributing to Olympia, so definitely in need of a review. :smile: 
does this need to be in the `<head>`? can we move it [near `</body>`](#diff-8944b57466f08564caa53a1988261ae0R155)?