Cell type to count and link reference-* relationships

[mike-hogan]

If a customer references many bookings, when I view a customer or a list of customers, I want to be able to have a cell that shows:

• this customer has three bookings
• this customer has no bookings
• this customer has five bookings

And when I click on link showing this information, I will go to the bookings view, with the list pre-filtered by the customer

This will require #20

[mike-hogan]

Unnecessary now that bi directional relationships are done