Add support for featured items in events

We want to extend events to allow flagging some wearables/emotes as featured items for the event.
Users creating events will be able to add a URN for a collection or for a specific item during the event creation.
In the client a small flag will show in the events list which events have featured items available.
In the event detail popup the list of featured items will be visible and will allow users to buy them via client.