Bug 15260: Modify next/prev_open_day

Bug 15260: Modify next/prev_open_day

We require next_open_day & prev_open_day to be more flexible. We could
create a separate sub, but that's not very DRY given that they'll do
pretty much the same thing.

So next_open_day becomes next_open_days and prev_open_day becomes
prev_open_days and both functions accept an additional parameter which
determines how many days they add or subtract.

All calls of these two functions have been modified accordingly.

Sponsored-by: Cheshire West and Chester Council
Sponsored-by: Cheshire East Council
Sponsored-by: Newcastle City Council
Sponsored-by: Sefton Council
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>