Add iCal export of rooms

[developeregrem]

Most of the booking portals like airbnb or booking.com support the synchronisation of calendars using the iCal format (calendar synchronisation).
Since this type of synchronisation is most of the type only supported for single room/apartment properties the iCal export function can be implemented per room in fewohbee. An option in the room management page must be added to allow the export of each single room.
A unique and publicly accessible URL must be generated for each room.