use left/right arrow keys to move between images in the web interface

[jeremybmerrill]

This PR adds a little bit of Javascript in the web interface to let the user use the left/right arrow keys to move between images. Left moves back in time; right moves forward in time.

(Prior to this PR, I was clicking from image to image, which was hard, requiring a context-shift from looking at cars to finding the right link to click, then back to looking at cars.)