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/integration/broken_deploy_test.rb

@@ -29,6 +29,5 @@ class BrokenDeployTest < IntegrationTest
       assert_match /First web container is unhealthy on vm[12], not booting other roles/, output
       assert_match "First web container is unhealthy, not booting workers on vm3", output
       assert_match "nginx: [emerg] unexpected end of file, expecting \";\" or \"}\" in /etc/nginx/conf.d/default.conf:2", output
-      assert_match 'ERROR {"Status":"unhealthy","FailingStreak":0,"Log":[]}', output
     end
 end