Batch download original files 12137

This addresses https://trac.openmicroscopy.org.uk/ome/ticket/12137

(Not ready for merging yet - just wanted to let @imunro try this out).
This PR is on-top of #2219 which should be merged first.

To test, select multiple images in web and try to download from toolbar in right panel.
Try some multi-file images and see how long it takes!
You are given the file count and size of files (before being zipped) in the download option.

Still TODO:
 - Fix image link when multiple images are selected.
 - Maybe open the download in a different window (so as not to freeze webclient)? Or at least show "waiting" while zip is being prepared.