Adds the FARSIDE_SERVICES_DIR environment variable to set the folder to
create/read the services.json file from.

The sourcehut link is (rightfully) blocking unauthenticated requests, so
using the github link is more reliable.

Services like searxng don't need to have instance checks performed since
the nightly cron task filters out the instances already.

The project was rewritten from Elixir to Go, primarily because:

- I don't write Elixir anymore and don't want to maintain a project in a
  language I no longer write
- I already write Go for other projects, including my day job, so it's
  a safer bet for a project that I want to maintain long term
- Go allows me to build portable executables that will make it easier
  for others to run farside on their own machines

The Go version of Farsside also has a built in task to fetch the latest
services{-full}.json file from the repo and ingest it, which makes
running a farside server a lot simpler.

It also automatically fetches the latest instance state from
https://farside.link unless configured as a primary farside node, which
will allow others to use farside without increasing traffic to all
instances that are queried by farside (just to the farside node itself).