Cleaning up display of slideshow preview images, including adding an .image class to the container for images (for easier CSS styling)

Signed-off-by: Matt Bernhardt <mjbernha@mit.edu>
Introducing some IE-specific fixes for the footer (display:table overrides for the flexboxes there);
This includes adding an IE9 conditional check to the existing IE7 and IE8 checks to header.php;
For the time being, I've also removed the position:fixed from the footer, because of an issue with the AddThis flyout.

Signed-off-by: Matt Bernhardt <mjbernha@mit.edu>