• Support "byweekday" rules for recurring bookings
• Add new UI widget to select weekdays

An example for such a recurring booking would be "weekly, monday+tuesday". Right now the only way to do this would be making two separate bookings, which is not user-friendly.

Related tasks for the Burotel CERN plugin (these do NOT need to be done at the same time, but depend on this being done in the core):

• Allow weekly repetition (currently: only daily repetition)
• Verify correct ADaMS sync (should still cover the whole period)