Include categories when fetching admin/web_hooks and make
'extras' more useful. 'extras' is the mechanism we use to provide
context for rest objects.
However, previously:
* When you fetched many objects, extras was only set on the ResultSet,
not on each object,
* If you need derived data from extras, there wasn't a sensible place to
put this code. Now, you can create an 'ExtrasClass' static field on
your rest model and this class will be used for your extras data,
