Clean up the /dashboard view in the UI

[ahellander]

Is your feature request related to a problem? Please describe. A couple of issues on the dashboard view that can be improved:

• The table is not particularly useful and will overflow the page for many metrics. Suggestion is to remove and replace with the option to download a csv or json file containtin that same data.

• The client training time distribution is useful, but the pie chart shows the average over all clients and is less interesting. Suggestion to remove the pie chart and instead only show the histogram (and fix the legend).

• Add some explanation of how to interpret the histogram.

• Top plot (replace x axis "Rounds" with "Global round" to avoid ambiguity.