Fix "The operation is insecure." when trying to register fido2 device.

navigator.credentials.create(); Doesn't accept a port in the "id" parameter. So, when trying to register a fido2 device via WebAuthn throws: "The operation is insecure." on firefox and "The relying party ID is not a registrable domain suffix of, nor equal to the current domain." on Chrome or Edge.
This commit replaces `$_SERVER['HTTP_HOST']` with `$_SERVER['SERVER_NAME']` when initializing `$WebAuthn` which excludes the port to formulate correct requests.
Now Mailcow allows the registration of fido2 devices when running in a non-standard port(eg. 443).