Write a Borders API which makes use of topology

[joshbrooks]

Description

Area boundaries, which we use in a few places in DIRD, are not well used at the moment. For instance there's a 500kb+ download on the Project List of all areas

which when zoomed in looks like this

On the 'national map' we hit that same endpoint iirc but with different areas

What went wrong?

We're abusing the data.geoson endpoint in order to fetch these area boundaries. Rather than doing so, we could fetch topoJSON which should be around 10% - 30% of the size depending on simplification.

Additional Data

This depends on the borders table generation in release 3.1.2