feature request: add some easy way of visualizing areas returned by /areas queries

[mhl]

This would be useful for the Democratic Commons project's audit of boundary data, which includes trying to see how much boundary data that that we'd need is already complete / accurate enough in OpenStreetMap. The idea is to make it easy to get a quick at-a-glance sense of what the coverage at a particular admin level is for given countries in global.mapit . For example, we might compare this visualization with other maps of the administrative boundaries for a country. (In order to actually do this, we also need to merge and deploy #302, and wait for the job which is currently running to set the country on all areas to complete.)

This could be a simple Leaflet-based page that takes a MapIt JSON results URL from /areas, fetches a GeoJSON file for each of areas in those results, and displays it on the map. However, I think this is useful enough that perhaps it's worth building into MapIt - my idea was that as well as the .html extension for the text-based results for multiple areas, you could also have .map.html, which would render the areas returned on a map.

I guess we would probably want to limit the number of areas that would be fetched, since, for example, queries for all areas whose name starts with a would produce a browser-crashing number of areas to render.