Replace Traefik with parachute

[mproxy](https://github.com/basecamp/parachute) is a custom minimal
proxy designed specifically for Kamal.

It has two big advantages over Traefik:
1. Imperative deployments - we tell it to switch from container A to
   container B, and it waits for container B to start then switches. No
   need to poll for health checks ourselves or mess around with forcing
   health checks to fail.
2. Support for multiple apps - as much as possible, configuration is
   supplied at runtime by the deploy command, allowing us to have
   multiple apps share an instance of mproxy without conflicting config.

test/fixtures/deploy_workers_only.yml

@@ -2,7 +2,7 @@ service: app
 image: dhh/app
 servers:
   workers:
-    traefik: false
+    proxy: false
     hosts:
       - 1.1.1.1
       - 1.1.1.2