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/proxy_test.rb

@@ -0,0 +1,66 @@
+require_relative "integration_test"
+
+class IntegrationProxyTest < IntegrationTest
+  test "boot, reboot, stop, start, restart, logs, remove" do
+    kamal :server, :bootstrap
+    kamal :envify
+
+    kamal :proxy, :boot
+    assert_proxy_running
+
+    output = kamal :proxy, :reboot, "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-proxy-reboot", "post-proxy-reboot"
+    assert_match /Rebooting proxy on vm1,vm2.../, output
+    assert_match /Rebooted proxy on vm1,vm2/, output
+
+    output = kamal :proxy, :reboot, "--rolling", "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-proxy-reboot", "post-proxy-reboot"
+    assert_match /Rebooting proxy on vm1.../, output
+    assert_match /Rebooted proxy on vm1/, output
+    assert_match /Rebooting proxy on vm2.../, output
+    assert_match /Rebooted proxy on vm2/, output
+
+    kamal :proxy, :boot
+    assert_proxy_running
+
+    # Check booting when booted doesn't raise an error
+    kamal :proxy, :stop
+    assert_proxy_not_running
+
+    # Check booting when stopped works
+    kamal :proxy, :boot
+    assert_proxy_running
+
+    kamal :proxy, :stop
+    assert_proxy_not_running
+
+    kamal :proxy, :start
+    assert_proxy_running
+
+    kamal :proxy, :restart
+    assert_proxy_running
+
+    logs = kamal :proxy, :logs, capture: true
+    assert_match %r{"level":"INFO","msg":"Server started","http":80,"https":443}, logs
+
+    kamal :proxy, :remove
+    assert_proxy_not_running
+
+    kamal :env, :delete
+  end
+
+  private
+    def assert_proxy_running
+      assert_match %r{registry:4443/basecamp/kamal-proxy:latest   "kamal-proxy run"}, proxy_details
+    end
+
+    def assert_proxy_not_running
+      assert_no_match %r{registry:4443/basecamp/kamal-proxy:latest   "kamal-proxy run"}, proxy_details
+    end
+
+    def proxy_details
+      kamal :proxy, :details, capture: true
+    end
+end