Returned slideshow to JS. Removed Sass mixin slideshow.

Used a different JS slideshow than was in our original site because it seemed straightforward, however:

**Pro.** Provides for more semantic and accessible images because the images are in `img` tags.
**Con.** Because the images are not in a `background-image` property, it is difficult to crop/resize them based on screen width. This may come to a head when trying to address the third point in #219 .

Closes #192 .