Update Schedule.php to exclude holidays from iCal schedules

[natep-tech]

Completes #344

This takes an array of DateTimes that should be excluded from the schedules, like University holidays (calendar link here), and adds it to the generated iCal files so that class events will not be created on those days off. Using the iCal spec EXDATE, we can skip those days and make sure we don't confuse students.

BLOCKERS:

It is up to you to source the array of DateTimes to exclude. I have named the placeholder variable $holidayTimes, but you must provide the list of dates before this can be merged and implemented.