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.
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