Departmental leaderboard

This is an inverse leaderboard ordered by descending order of number of errors per department, across all their sites. This is intended to coax them into fixing their mappings to get lower down in this list. :sunny:

This was originally meant to be a spike, and I've done it as a spike, but I also wrote some tests for it and got a fair amount of feedback as you'll see in the comments. I'd quite like to preserve all of this work, as it seems decent (although obviously I'm biased), hence the PR.

- Do we want to limit the leaderboard to, say, 20 departments? It's currently ordered by descending number of errors, which is good (? I think that's what Dave intended), but it does list every department so it's quite a long page.
- Can this have @fofr's :eyes: please?
Rather than a down arrow, did you try a Bootstrap icon? perhaps `glyphicon glyphicon-arrow-down`:
http://getbootstrap.com/components/