Slideshow fixes

This pull fixes a few things:
1. Proper handling of IDs (from CF's work)
2. Eliminates bug where # of images was capped at 5
3. Honors link="file" shortcode attribute, linking outputted image to original JPG (in new window)

The JS library that powers the slideshow is actually very out of date, but it still works so I didn't touch it. Upgrading it would require an extra hour or two, as we'd need to retool navis-slideshows.js a bit to pass new settings into the library.