Item page lightbox issues

[zerocrates]

The lightbox feature added in b850c73 has some problems:

1. it doesn't account for the theme setting on whether the gallery links to the file or the file show page: when linking to the show page the lightbox won't work
2. it doesn't account for non-image files, e.g. audio

There's a branch revert-lightbox that just takes this commit out. I don't know if you want to go that way or make other changes.