Use templates for SOAP requests

See [review comment](https://github.com/oracc/nisaba/pull/64#pullrequestreview-736307195):
> The only thing I'd consider which might make things easier to understand is to use templates for the XML text rather than chunks of formatted text inbetween lines of code.

There are [several options for templating engines](https://colorlib.com/wp/top-templating-engines-for-javascript/).