Maintenance mode

Adds support for maintenance mode to Kamal.

There are two new commands:
- `kamal app maintenance` - puts the app in maintenance mode
- `kamal app live` - puts the app back in live mode

In maintenance mode, the kamal proxy will respond to requests with a
503 status code. It will use an error page built into kamal proxy.

You can use your own error page by setting `error_pages_path` in the
configuration. This will copy any 4xx.html or 5xx.html files from that
page to a volume mounted into the proxy container.

test/fixtures/deploy_with_error_pages.yml

@@ -0,0 +1,11 @@
+service: app
+image: dhh/app
+servers:
+  - "1.1.1.1"
+  - "1.1.1.2"
+registry:
+  username: user
+  password: pw
+builder:
+  arch: amd64
+error_pages_path: public