Adds flexbox layout for centering small pictures in slideshows that are ...

...larger than some of the content.

For example, if a slideshow on an iMac is 1,000px wide, and the photo within the slideshow is only 500px wide, that photo will be centered, instead of sitting off to the left. Similarly, an image that is too small vertically will be centered along the y-axis instead of sitting at the top of the slideshow.

Browsers that don’t understand flexbox will continue to function as before.