Add 'Export data' button which delivers JSON as downloadable file

[AlanSl]

The human-readable data is contained as JSON within the HTML whereas the data dump file written to the server is binary. Users may want to view the data, so let's make it easy for them. Since this data is already in the HTML file it shouldn't increase file weight and should be very easy to implement.

I'd say JSON only is fine, a CSV option might be a nice-to-have but I'd say it's not a priority since the kind of users who will be exporting data files are probably also the kind of users who can convert JSON to CSV themselves if that's what they needed (and maybe makes more sense server-side as a CLI option so it can be used with --collect-only?).

Regarding where in the UI, I'm also drawing up some suggestions for a feature to import data to allow different Doctor benchmarks to be compared side-by-side: it'd make sense for import/export buttons to be together, and the most intuitive place for these would be the very top header aligned right opposite the Doctor logo since it's a step outside the context of the current analysis and recommendations.