[Feature] Support for Split schedules

[MoralCode]

Some schools have "split" schedules where one group of students goes to "early lunch" while the other is in class, and then things swap. This swap usually does not happen at the same time for the students in the early lunch group as it does for the later group.

ClassClock should ideally support this type of schedule.

Rough implementation thoughts

I suspect one good way to implement this is to introduce another field to each class period within a bell schedule that assigns that item one or more "variant" identifiers. this way the schedule can contain overlapping time entries and the web app can easily disambiguate them and show a prompt to ask which schedule the user would like to select (possibly remembering this in browser storage for the future)

Possible Challenges

• displaying this on the Admin UI in a sensible way
• handling students whose lunch schedule changed based on the day (i.e. based on who their teacher is that day)
  • this may require adding support for tracking A/B days as well.... which would be a lot of extra work
• ensuring that all aspects of the webapp are able to handle these "variations"