Requested due dates should exclude calendar exception due dates

[#1]

Requesters should not be able to choose due dates which have been designated as "Exception" dates on calendars. Right now, only TASK due dates are impacted by the exception dates that the Admin sets up. This exclusion needs to happen at the REQUEST level so that when a requester chooses one of those dates, an error message appears advising that the date is a "holiday", "weekend" or some other "exclusion" date, and that they need to choose another date.