Don't allow the user to update capacity if capacity is lower than the number of current attendees.

[fede-rodes]

Similarly, we need to be careful when the user attempts to update the game capacity. We should complain in case the new capacity is lower than the number of current attendees. For instance, let's say the user wants to pass from 10 to 8 players (capacity), but there are already 9 players on the list of attendees, I think we shouldn't allow for this to happen; the organizer should remove one player before the update.