Add option to redirect to main URL

[bensofficial]

This change adds the option to add a main URL. If the main URL is defined, then all requests will be redirected to the main URL.

[kliwniloc]

You may want to support the use case of having another separate SSL certificates for the old and the new domains. You could set the existing certificate as the default for both.

[kliwniloc]

Furthermore, I recommend moving common SSL parameters into a distinct file to keep it maintainable. For guidance, consult the NGINX include documentation or review a practical tutorial by Digital Ocean.