flag_overdue task only works if it runs every day

[orenyk]

Currently, the Rake task flag_overdue only looks for Reservations whose due dates were yesterday and so if it doesn't run one day those reservations will never be flagged as overdue. Instead, we should search for reservations whose due dates were yesterday or earlier and still have the status of checked_out so that one missed cron job (or a period without cron jobs) doesn't prevent proper reservation flagging.

Note that this should probably be done for flag_missed as well.

[orenyk]

flag_missed already has this set, so it's just a matter of changing flag_overdue. I'm blocking this on #1273 since it requires editing the same file.

[orenyk]

I rebased the branch for this issue onto the new one for #1273 and will wait to open a PR until #1277 is merged.