Node mailer util

[PaulioRandall]

What does this change?

Adds the postMail function to the utils package that posts mail via node-mailer instance.

Why?

Because the API is easy to forget and error handling is non-standard. It's doubles up as documentation for sending requests to node-mailer.

How is it tested?

Used and tested in https://dev.ldn-gis.co.uk/cool-spaces/

How is it documented?

TypeScript and file comments.

Is it complete?

• [x] Have you included changeset file?
• [x] If this adds a new component, is it exported via index.js?

[PaulioRandall]

Not needed, push to template instead.