Late fee calculation should be a model method

[orenyk]

Currently, we calculate the late fee for a given reservation in the relevant view (/reservations/:id) and in the relevant e-mail body; this logic should be in the model method and it should be dependent on whether or not the reservation is checked-in (e.g. either calculate estimate based on today's date if not checked in or calculate final fee).

[orenyk]

Ha, this was resolved in #1121 / #1197 by @squidgetx 😄. Closing!