Ensure benchmark_copy results persist

[Chris Warburton]

So far we're copying to /opt/benchmarks, that's fine. We need to make
these accessible to the Web server. I've done this by symlinking
/var/www/html/benchmarks to /opt/benchmarks. This seems fragile: in
particular, we should look at how the git symlinks are currently set up
for chriswarbo.net, and add the benchmarks to that.