Bug 21727: Add handling for cases requireing patron refunds

Bug 21727: Add handling for cases requireing patron refunds

As it is possible to partially pay off a fine that is still incrementing
we can end up in a position (when dropbox mode is enabled) where the
patron may be owed money by the library should they have paid off the
fine between dropping the item off in dropbox mode and the increment
that reverts the increase on the dropbox day.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>