Replace Traefik with kamal-proxy

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

It has some 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 a proxy without conflicting config.
3. First class support for Kamal operations - rather than trying to
   work out how to make Traefik do what we want, we can build features
   directly into the proxy, making configuration simpler and avoiding
   obscure errors

test/integration/app_test.rb

@@ -1,16 +1,18 @@
 require_relative "integration_test"
 
-class AppTest < IntegrationTest
+class IntegrationAppTest < IntegrationTest
   test "stop, start, boot, logs, images, containers, exec, remove" do
     kamal :envify
 
+    kamal :setup
+
     kamal :deploy
 
     assert_app_is_up
 
     kamal :app, :stop
 
-    assert_app_is_down
+    assert_app_is_down response_code: "504"
 
     kamal :app, :start
 
@@ -24,7 +26,7 @@ class AppTest < IntegrationTest
     logs = kamal :app, :logs, capture: true
     assert_match /App Host: vm1/, logs
     assert_match /App Host: vm2/, logs
-    assert_match /GET \/ HTTP\/1.1/, logs
+    assert_match /GET \/up HTTP\/1.1/, logs
 
     images = kamal :app, :images, capture: true
     assert_match /App Host: vm1/, images
@@ -50,6 +52,6 @@ class AppTest < IntegrationTest
 
     kamal :app, :remove
 
-    assert_app_is_down
+    assert_app_is_down response_code: "504"
   end
 end