[UP-4149] Update the create tenant workflow to send a custom email inste...

...ad of using the reset-password email.

Moved the guts of the code to generate and send that email to a more generic notification interface so that I could wire up 2 different impls and potentially eventually support other notification methods (eg. SMS).
Public fields -- would be good for these to have JavaDoc, ideally with `@Since uPortal 4.1`.