Multi-file and folder download

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This may be slow in reality. I would suggest using it for only downloading a batch of small files in large quantities, instead of a couple of large files.

We use JSZip to download all files in-browser and compress them into file blobs, which are then downloaded to your device as a zip file.

This is extremely useful if a series of small files are to be downloaded, but it could be time consuming if used for bulk-downloading a few large files.

Folders are traversed and recursively fetched into the compressed zip. Multiple files or folders can be selected and downloaded side-by-side.

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Discussed in PR #177 and #169.