Add a `ticket_drop_time` to control ticket visibility and editing

[rohangpta]

create_tickets currently deletes and recreates tickets / other parameters around the event every time it is called

This could be very problematic if a bought ticket is deleted since the organizer decides to go in and add a few more tickets.

We should:

• Add a parameter to events specifying a ticket_drop_time
• Make tickets publicly available for purchase only after this time
• Freeze ticket offering editing after this time