Revert "Merge pull request #2373 from openscholar/2352-slideshow_scaling...

..."

This reverts commit 383156ca7bdcad481b15446254ac69eb1eb1ff50, reversing
changes made to 77210294d741ad1dd23835b97b4894e3415ffbf0.

#2352 fixed the image cut off in the slideshow.  But it would be problematic for existing slideshows.  Instead of breaking those, we should tell users that there's a new slideshow on the way and until then, they can paste portrait images on a wider background to make them landscape.