Add support for vacation and holidays

[make-github-pseudonymous-again]

At the very minimum it could just be the creation of events that span the entire day. They would be stored as intervals from 00:00 to 23:59 and displayed with custom logic to span the planned work schedule.

Currently, the user's workaround is to add a long duration to possible appointment durations for a synthetic "vacation" patient, then create multiple events that cover the planned work schedule. It is inconvenient because passed vacation is registered as no-shows for that synthetic patient, which makes past vacation difficult to browse in the agenda itself. You can easily browse past and future vacation through the synthetic patient history though.