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,
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
