Cross-event Discount codes

2 events of the same organization should be linkable.
each link is unidirectional and define a discount amount (first --> second)
2 links (one for each direction) can be defined

When a user buys a ticket for the second event, he can insert the ticket UID of the first event in the field promotional code and the discount will be applied.

The first ticket will become a "parent" ticket
The second ticket will become a "linked" ticket

Still to be discussed if we want to implement this:
if a parent ticket is re-assigned we should ask if the same change has to be applied for the linked tickets. all re-adssigned tickets will be re-send to the destination email. if answer no... we display a message saying "This is not fair"
If a linked ticket is re-assigned to another person, we display a message saying "This is not fair"