Rewrite layer build system in python

[jacobwhall]

• Re-write build system in Python
• Automatically run layer builds for each area
• Allow configuring areas per-layer, for e.g. one surveillance layer for Greater Richmond?
• Load every build artifact into site using Mustache
• Reconfigure GitHub Action to build using new system

I also noticed that osm2geojson was parsing closed ways as areas, even if they aren't tagged that way. I'll have to address that before merging.