Disconnecting a calendar shouldn't delete a schedule

Just had an experience where I:

1. Downloaded my data (lucky me)
2. Disconnected my CalDAV
3. Went to Dashboard noticed the quicklink url was super long (another bug)
4. Adjusted it and clicked saved
5. Got the error message that "Schedule was not found"

I believe we technically require a schedule to have a calendar id, so in this case it was probably an intended path to delete any dangling calendar references. But that doesn't feel right. We should (if it's not already) make calendar_id nullable, and increase the "is active" condition on the schedule to include "if there's no calendar id selected."

It's also possible that this is just the database cascade triggering. Need to dig a little more into this at some point.

Reproduce the reported sequence: download data, disconnect CalDAV, then edit and save the Dashboard quicklink. Trace how disconnecting handles the schedule's calendar_id and check whether a database cascade or the active-schedule condition causes “Schedule was not found”; done means the schedule remains usable after its calendar is disconnected.