Batch download original files 12137

This addresses https://trac.openmicroscopy.org.uk/ome/ticket/12137

This PR is on-top of #2219 which should be merged first (or I guess it could be closed if we merge this).

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.