Server can send both plain-text and HTML templated email

[francisli]

The contents of the current para invite email is hardcoded into the source file. I think we should pull out email into separate template files, create both plain-text and HTML email bodies, and extract a common HTML layout template for styling.

Will look in to adding this email templating library I've used: https://www.npmjs.com/package/email-templates

And a tested open source HTML email template layout, such as: https://github.com/leemunroe/responsive-html-email-template