Avoid overbooking

[alvyynm]

• Ensure that the number of seats users reserve doesn't exceed max_attendees in the selected event.
• Implement locking to prevent race conditions
• Prevent shrinking of max_attendees below old.max_attendees - available_seats (i.e prevent user from reducing max_attendees below already booked seats, which will mean the event is overbooked)

[alvyynm]

#13 partially addresses the first item on the checklist