Use Schematic-generated heroku-go for API access

Replaces Blake's outdated heroku-go with the Schematic-generated one
which seems to have consensus as the way forward for Heroku API access
in Go.

Strips out logic for reading from `~/.netrc` and instead prefers the
more explicit system of having a `HEROKU_ACCESS_TOKEN` passed in on
start-up.

@fdr I'm probably just going to start merging stuff here. If you disagree with
something in particular, let me know, and we can go back and work through it.