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/main_test.rb

@@ -1,14 +1,15 @@
 require_relative "integration_test"
 
-class MainTest < IntegrationTest
+class IntegrationMainTest < IntegrationTest
   test "envify, deploy, redeploy, rollback, details and audit" do
+    kamal :server, :bootstrap
     kamal :envify
     assert_env_files
     remove_local_env_file
 
     first_version = latest_app_version
 
-    assert_app_is_down
+    assert_app_is_down response_code: "502"
 
     kamal :deploy
     assert_app_is_up version: first_version
@@ -28,11 +29,11 @@ class MainTest < IntegrationTest
     assert_app_is_up version: first_version
 
     details = kamal :details, capture: true
-    assert_match /Traefik Host: vm1/, details
-    assert_match /Traefik Host: vm2/, details
+    assert_match /Proxy Host: vm1/, details
+    assert_match /Proxy Host: vm2/, details
     assert_match /App Host: vm1/, details
     assert_match /App Host: vm2/, details
-    assert_match /traefik:v2.10/, details
+    assert_match /basecamp\/kamal-proxy:latest/, details
     assert_match /registry:4443\/app:#{first_version}/, details
 
     audit = kamal :audit, capture: true
@@ -45,11 +46,12 @@ class MainTest < IntegrationTest
   test "app with roles" do
     @app = "app_with_roles"
 
+    kamal :server, :bootstrap
     kamal :envify
 
     version = latest_app_version
 
-    assert_app_is_down
+    assert_app_is_down response_code: "502"
 
     kamal :deploy
 
@@ -79,7 +81,6 @@ class MainTest < IntegrationTest
     assert_equal({ user: "root", port: 22, keepalive: true, keepalive_interval: 30, log_level: :fatal }, config[:ssh_options])
     assert_equal({ "multiarch" => false, "args" => { "COMMIT_SHA" => version } }, config[:builder])
     assert_equal [ "--log-opt", "max-size=\"10m\"" ], config[:logging]
-    assert_equal({ "path" => "/up", "port" => 3000, "max_attempts" => 3, "cord"=>"/tmp/kamal-cord", "log_lines" => 50, "cmd"=>"wget -qO- http://localhost > /dev/null || exit 1" }, config[:healthcheck])
   end
 
   test "setup and remove" do