Add late fee cap

[orenyk]

Requested by CSSSI (I think):

We have decided we won’t charge more in fines than the item would cost to replace, but the system is calculating an amount based on number of days and daily fine charges.

We could add an extra option for equipment models (e.g. late_fee_max) that would default to zero (no cap), but allow admins to specify a maximum late fee for a given model. Since we moved final late fee calculation to the Reservation model (late_fee), we can easily incorporate this logic there without changing anything else that expects a late fee (e.g. various views, e-mails, etc). Thoughts?

[orenyk]

Ok, all done. This was based off of the #1286 branch (mis-named 1287_...) which was based off of the #1284 branch; as such, it's blocked on the merging of #1287 and the PR for #1286.