Recurring bookings with "byweekday" repetition

[ThiefMaster]

• 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)

[ThiefMaster]

As discussed offline, this should be an extension to the "recurring" booking. Below the "every [n] [weeks|months]" there would be a button group (or similar) that shows the 7 week days, with the one of the start day always being selected (since it would be pointless to exclude it). You can then add additional weekdays if you want, and those would be included in the booking.

On the server side this would simply affect the rrule. You likely need a new column (postgres array of strings?) to store the weekdays. for this one I think nullable=true makes sense (and keeping it null for single/daily bookings, maybe a check constraint to enforce this).