ImagePicker upgrade

Test Plan:
Run tests
Verify that imagepicker doesn't disappear on mouseout and the imagepicker lazy loads images
JS ImagePicker upgrades

Test Plan:
Run tests
Verify that imagepicker doesn't disappear on mouseout and the imagepicker lazy loads images

Reviewers: pamela, john, brian

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14114