Add a plot of the FOAF link-length distribution to the statistics page.

This proposal adds a linear link-length distribution plot for friend-of-a-friend links to the statistics page. The plot includes a histogram (green) and a cumulative histogram (grey). This eliminates the need to run probe requests to quickly and roughly estimate the link-length distribution in the local neighbourhood.

The rationale behind using FOAF links and not local links is that we expect the FOAF links to be numerous enough to provide a reasonable estimate of the neighbourhood, where the local links don't give much information at all (especially on low-bandwidth nodes).

This could prove useful in the assessment of success of topology changes, such as those described in ```USK@ZLwcSLwqpM1527Tw1YmnSiXgzITU0neHQ11Cyl0iLmk,f6FLo3TvsEijIcJq-X3BTjjtm0ErVZwAPO7AUd9V7lY,AQACAAE/fix-link-length/1/```.
I'm not able to find a reason why FOAF locations must be valid. It'd make sense to me to skip invalid locations here.

It took me a while to get that the `* 2` was to scale from the max 0.5 distance to the full histogram length.