Fix double bookings

What does this MR do?

Manipulating the index variable allowed signed in guests to change their booking index and thus manipulate the slot they would like to book, possibly resulting in double bookings. This change makes sure no booking can result in a double-booked slot.

Closes #144

Does this MR meet the acceptance criteria?

• I have added tests to validate the fix.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text.