refs https://github.com/TryGhost/Toolbox/issues/292
- There's a need to reuse these utils in the version mismatch notification service. Having loads of tightly coupled dependencies makes it super hard to rip out this module for reuse
- It's a groundwork for extraction of the email-utils package
- Rewrote the unit tests that were written for these utils previously - they weren't testing anything useful. The goal of this util is to generate specific content based on provided data and available templates - now the tests do test those specific things, not the mailer itself!
