Remove old-style gallery lightbox and thumbnails

This code relates to the old gallery thumbnail mosaic and lightbox, both of which have since been replaced. I think this code is hanging around because old-style containers allowed you to open a gallery lightbox by clicking on a link on a front, which is no longer the case.

Is it safe to delete this stuff now, @mattosborn?

(Separately, should `common/app/views/fragments/trailblocks/` and `common/app/views/fragments/trails/` be removed too? Do these relate to old style containers too? Removing these seemed a bit more dangerous, so something for a separate PR!)

/cc @phamann