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/integration_test.rb

@@ -44,10 +44,10 @@ class IntegrationTest < ActiveSupport::TestCase
       deployer_exec(:kamal, *commands, **options)
     end
 
-    def assert_app_is_down
+    def assert_app_is_down(response_code: "503")
       response = app_response
-      debug_response_code(response, "502")
-      assert_equal "502", response.code
+      debug_response_code(response, response_code)
+      assert_equal response_code, response.code
     end
 
     def assert_app_is_up(version: nil)
@@ -101,8 +101,8 @@ class IntegrationTest < ActiveSupport::TestCase
     def assert_200(response)
       code = response.code
       if code != "200"
-        puts "Got response code #{code}, here are the traefik logs:"
-        kamal :traefik, :logs
+        puts "Got response code #{code}, here are the proxy logs:"
+        kamal :proxy, :logs
         puts "And here are the load balancer logs"
         docker_compose :logs, :load_balancer
         puts "Tried to get the response code again and got #{app_response.code}"
@@ -129,10 +129,10 @@ class IntegrationTest < ActiveSupport::TestCase
     def debug_response_code(app_response, expected_code)
       code = app_response.code
       if code != expected_code
-        puts "Got response code #{code}, here are the traefik logs:"
-        kamal :traefik, :logs
+        puts "Got response code #{code}, here are the proxy logs:"
+        kamal :proxy, :logs, raise_on_error: false
         puts "And here are the load balancer logs"
-        docker_compose :logs, :load_balancer
+        docker_compose :logs, :load_balancer, raise_on_error: false
         puts "Tried to get the response code again and got #{app_response.code}"
       end
     end