Add support for affiliate id

[jasontorres]

In some cases that we have widgets being embedded on a 3rd party site, an event organizer can give an affiliate id to the 3rd party site owner for the organizer to track their referrals.

The current parameter is ?aff=N, where N is the affiliate id.

This is related to the new work we've added within Ticketbase itself for event-based affiliates.