Download preserves directories of original files

This fixes the loss of directory structure in downloaded zips as reported by @dpwrussell using volocity files.

To test:
 - Try to download any MIF format that has data in different directories, E.g. volocity.
 - The zip should contain all the MIF files, with only their common path removed.
 - This approach is used for all downloads, so if you select 2 images imported on different dates, you will get a download zip based on their common parent directory, E.g.
```
28/11-34-35.964/Fig\ 4.mvd2
28/11-34-35.964/Data/... (volocity data)
14/14-02-52.544/tubulin1.tif
```

It might be possible to remove some unnecessary directories, but we could introduce bugs - safest is to preserve all the divergent paths that are in the Managed Repository. 
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