Slideshow opened from files view now loads images from gallery (fixes orientation)

When opening the slideshow from the files view, the image URL is
converted into a gallery URL. This makes sure it uses the cache and
correctly rotated imaged.

Previously it was using the download URL.

Fixes #1462

I've also tested with shared pictures and it works as well!

Please review @jancborchardt @butonic @icewind1991 @karlitschek