[build] pin artifacts in a committed manifest

[michaelkirk]

I'm primarily attempting to solve two problems.

currently

1. Promoting transit graphs used to happen out of band, by symlinking a transit graph to "latest". This was manual, and meant rollback was also manual and happened outside of git/k8s.
2. Generating k8s configuration, in particular the transit zones, reflected on an env variable. But this state was also duplicated by the presence of *.gtfs_feeds.csv, and to complicate things there were various capitalization conventions (some were camel case, some were kebob case)

this change

1. Now: the versioned transit graph (with date) is baked into the k8s configs.
2. stronger naming conventions for the transit zones obviates any dynamic name mangling.
3. The k8s configuration is generated from a new manifest ("artifacts.json") which is output as part of the build process and should be committed.

There is no longer a HEADWAY_TRANSIT_AREA's env variable. The build process infers which transit zones to build based on the presence of a gtfs_feeds file alone.

When the build completes, those (and the other build artifacts) will be in the artifacts.json, which is then used to drive bin/publish-artifacts and bin/k8s-generate.

It's not less code... so calling it "simpler" is a stretch, but it's not really any more code, it dedupes some state, and decouples more parts of the build->deploy process.

I think it's a win overall, but the proof will be in how it feels after a couple of build/deploy cycles.