Create some example extensions for the item overview

[Nicklas Nordborg]

BASE 3.2 will add extension points for adding node loaders and validators to the item overview (​http://base.thep.lu.se/ticket/1680 and ​http://base.thep.lu.se/ticket/1681). We should include some examples in the example code.

1. Include the user that is the owner of an item as a property node.
2. Continue to load the role membership of that user as child nodes.
3. A validator that checks if the user has a valid URL address or not.
4. A validator that checks if the user is a member of at least one role.