URGENT: Add additional protections for deleting events.

Problem: If a past event is deleted - particularly a large event with many attendees and payments - it is catastrophic since there is no option to restore an event and all of it's related invoices.

The Duplicate button and the Delete button both lie under the Edit menu and are easily confused.

Solution 1: INADEQUATE - Move the Duplicate and Delete buttons to different menus

Solution 2: ADEQUATE - Create a BIG RED warning when deleting a past event with a second "are you really sure you want to delete this event?" warning.

Solution 3: ADEQUATE - Disallow deleting an event unless and until ALL related invoices have been manually deleted or voided.

Solution 4: ADEQUATE - Allow undeleting of events including un-voiding of all associated invoices.