Allow to add stream entry

## Feature request

### Feature description

As we have **Add site** button, it would be nice to have **Add stream** button. File **nginx.conf** will look like:

```nginx
// ...
http {
// ...
include /etc/nginx/sites-enabled/*;
}
stream {
// ...
include /etc/nginx/streams-enabled/*;
}
```

### How the feature is useful

It is really good for reverse proxy. I run all apps as docker containers and some of them are not a http. For example git repository is ssh. Previous I had to expose container port to public `host_port_public:container_port`, with "stream reverse proxy" I can keep my containers not public `127.0.0.1:host_port_local:container_port`.

My **nginx.conf** looks like:

```nginx
// ...
http {
// ...
}
stream {
upstream ssh {
server 127.0.0.1:host_port_local;
}

server {
listen host_port_public;
proxy_pass ssh;
}
}
```

I don't know if there are some general configs for streams like for http (nginxconfig.io/*) or each stream has to have individual settings.

Start by tracing the existing Add site flow and how it produces nginx.conf. Compare that flow with the requested stream and streams-enabled include blocks, and determine which general and per-stream settings the tool should support. Done means the tool can add a stream configuration without breaking existing HTTP site generation and produces valid NGINX configuration for the stated reverse-proxy case.

Written by the indexing model from the issue text.