[Events] Support for multiple prices for single event (Tiered pricing)

[JakobTopholt]

Events which share the same max amount of tickets sold, but differ in price and ID, but when runningunning an SQL query for participants on ID can generate all.

This way an event may have partical tickets and full tickets. Such as F-Sportsdag where some members might not want to participate in the "afterparty"

Perhaps just a checkbox in the Django interface for products, in which it be be checked that a product is a subclass of a superclass..