Cache majority of OSM API requests

[jakecoppinger]

To improve first load speed and reduce load on the OSM API, we could download the node locations at build time.

I like the idea of building almost as a "static cache", and if there's a cache miss we still hit the API (so new measurements still load between builds and deployments)

[jakecoppinger]

Implemented in 63ad931

However I'm seeing OSM node disappear more often than I expected would happen. I'll make an issue to track a fix for storing coordinate locations as well as OSM nodes to be able to find node replacements manually if necessary.