Create new setting for booking reason required

[moliholy]

Closes #6150

Room booking: reason required configuration

This PR allows to set a configuration parameter to determine whether specifying a reason for a booking is mandatory or not.

Screenshots

[moliholy]

@ThiefMaster ready for review!

[ThiefMaster]

Some issues I noticed while testing:

• There's an empty "Booking reason" box when viewing a booking. I already pushed a fix for this.
• When the reason is optional and you start entering a reason and delete it again, creating the booking fails, because we send '' instead of omitting the field. I already pushed a fix for this.
• There's no server-side enforcement of this setting. Not really a big problem considering one could simply put xyz or whatever in the reason. Still better if you could fix it.
• When editing a booking, the reason field is still always required. The behavior there should match the one we have when creating a new booking.

[moliholy]

@ThiefMaster

There's no server-side enforcement of this setting. Not really a big problem considering one could simply put xyz or whatever in the reason. Still better if you could fix it.
When editing a booking, the reason field is still always required. The behavior there should match the one we have when creating a new booking.

Both of them are now addressed.

[ThiefMaster]

Needs a changelog entry :)

- Add setting to configure whether room bookings require a reason (:issue:`6150`, :pr:`6155`,
  thanks :user:`Moliholy, unconventionaldotdev`)

[moliholy]

Needs a changelog entry :)

Added 😄