Email notifications on new comments

- Some (tested) logic for building a list of recipients and a body
- Added the smtp-mail package but didn't get around to using it yet
- Various improvements to the testing harness
The code ended up structured a bit weird, but being able to have this kind of "end-to-end" test is pretty sweet, and the more I look at the module/function breakdown (especially what's `Handler`, `DB`, or pure), the more I like it.