Do not allow admin to change prices of TicketOption.price

[mbourque]

I feel it would be responsible design to disallow or warn the admin when changing the cost of one of the ticket options if there are existing tickets already sold. Same goes for delete ticket_option, which might indeed delete all associated tickets. Better to retire the active one.

The admin should be smart enough to know the outcome, but it would end up never being able to determine total ticket sales.