Add a relationship for linking artists to their tours

We can link artists to concerts (events) and concerts to tours (series), but we can't say whose tour it is, even though a tour is generally specific to a particular artist and it would make sense to be able to get to an artist's tours from their artist page.

For the link phrases I would suggest:

• Forwards: "tours"
• Backwards: "tour by"
• Long: "has tour"