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

...ad of using the reset-password email.

Tracked in [UP-4149](https://issues.jasig.org/browse/UP-4149).

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).