Lazy load imagemodal images

Test Plan: Check that all images appear in browser

Reviewers: pamela, john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13850
Factored out custom scrollSpy

Test Plan:
Check that nav updates when scrolling.
- Check that clicking nav links scrolls correctly, before and after first scroll

Reviewers: pamela, john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13858