Generated ical series not accounting for public and univiersity holidays

Thanks for the work done closing #63 in #343, I just tried out the new changes and it looks like it largely works!

I did notice that, because the first week of class starts with MLK day, it is causing all of my MWF classes to not generate until the next week


Additionally, it seems to be generating events through RIT's Spring break

since RIT's institute calendar also includes the public holidays that we have no class, this may be relatively easy to implement:

1. scrape, or get ITS to add an iCal version of the data from https://www.rit.edu/calendar
2. add some code to filter days off out from other institute events things (such as the last day of the add/drop period) and convert to DateTime format
3. follow the linked specs to include them as exception dates in each VEVENT declaration https://icalendar.org/iCalendar-RFC-5545/3-8-5-1-exception-date-times.html (thanks @natep-tech for the link)

Unless RIT can maintain the list of days off (which IMO it should) this may add some extra maintenance burden to CSH. However some of the open source groups at RIT may also be able to help take this on as well in the meantime.