Add setting to bind to a subnet instead of 0.0.0.0/::0

[MichaIng]

So one can configure the dashboard to listen on requests from LAN or localhost only, the latter e.g. for access via reverse proxy.

For access via reverse proxy, if one wants to use a sub directory instead of a sub domain, #113 is required as well, or internal links need to be generated based on the request URL instead of using hostname:port.