Reports: "get zip file" button

[oesteban]

Add a button that generates a zipfile and downloads it.

Supersedes #996

[oesteban]

https://stuk.github.io/jszip/

[oesteban]

Incidentally, I landed on https://stackoverflow.com/a/49836948

I think this should be doable by serializing the reportlets into the HTML and create a download blob. Potentially zipped with jszip, but not necessarily.

WDYT @rwblair ?