Replace Traefik with mproxy

[mproxy](https://github.com/kevinmcconnell/mproxy) 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/cli/healthcheck_test.rb

@@ -65,14 +65,14 @@ class CliHealthcheckTest < CliTestCase
     assert_match "container not ready (unhealthy)", exception.message
   end
 
-  test "raises an exception if primary does not have traefik" do
+  test "raises an exception if primary does not have a proxy" do
     SSHKit::Backend::Abstract.any_instance.expects(:execute).never
 
     exception = assert_raises do
       run_command("perform", config_file: "test/fixtures/deploy_workers_only.yml")
     end
 
-    assert_equal "The primary host is not configured to run Traefik", exception.message
+    assert_equal "The primary host is not configured to run a proxy", exception.message
   end
 
   private