Add new images to image picker, including Holiday tab

Summary:
This adds holiday images to image picker, making them really easy to find
via the Holiday tab. We can move them back into photos after HOC season.

Test Plan:
- Go to /cs/new/webpage, insert an image from Clipart tab, see nav-pills on side
- Insert an image from Photos tab, see nav-pills on side
- Insert an image from Holiday tab, don't see nav-pills on side

Reviewers: alexrodrigues

Reviewed By: alexrodrigues

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14450