Disable late checkin e-mail when late fee is zero

[orenyk]

We've been asked on several occasions to disable the late checkin e-mail, generally in cases when the client doesn't implement late fee functionality. We currently don't offer a way to do so (and I don't want to add clutter to our App Config page) but we could easily modify the user mailer method to only send that e-mail then there actually is a late fee. Currently sticking in the wish list but we can try to squeeze it in v4.2.0 if there's time.

[orenyk]

Felt like doing something small, here it is! :-P

I put the logic in the mailers for now; I felt like this made more sense than putting it in the Reservation model both from a testing perspective as well as an implementation perspective. Thoughts?