Model classes should have a resource property

[Ryan J Ollos]

The Changeset, Milestone and Node classes have a resource read-only property while the Attachment, Ticket, Repository and WikiPage classes create the resource attribute in their __init__ method.

Having resource as a read-only property seems simpler since we don't have to worry about updating the resource attribute as the object changes. For example: tags/trac-1.0.1/trac/ticket/model.py@:252#L236​. As a read-only property, resource will be implicitly updated as the object changes.

With this change and the one in #11609, if we ever find it worthwhile to create a ModelBase class it will be clear that resource and realm should be abstract properties (decorated with abc.abstractproperty). I also wanted to make this change ahead of modifying the Component and Version classes in #1233 so that the pattern to follow is clear.