Allow Custom Host as Maintenance Host

[JustinBack]

Is your feature request related to a problem? Please describe.
Right now Port 9000 on Localhost is enforced, when Nginx UI is not listening on localhost the Reverse Proxy does not work.
Its also nice to have a per-vhost maintenance page for multiple tenants

Describe the solution you'd like
Ability to enter a custom host into the maintenance option, or disable the maintenance option altogether.

Describe alternatives you've considered
Hiding the maintenance button

Additional context

nginx-ui/internal/site/maintenance.go

Line 233 in 5d3f478

locationContent.WriteString(fmt.Sprintf("proxy_pass %s://127.0.0.1:%d;\n", schema, nginxUIPort))

[0xJacky]

In v2.1.0, we hide the maintenance button into the selector.