Booker bookings filter 'booked_by' needs to be a list

Booking filter 'booker.booked_by' needs to be a list not the current single value (defaulting to 'essm-sync').

Manual imports have lead to other accounts being used instead of 'essm-sync' so filtering by a list including 'essm-sync', 'ras235' and 'Student Registry' will be needed.