Workoutset crud ajax

The front end javascript is pretty rough still, but I have added drag and drop re-ordering for the workoutsets.

Included in this:
* Drag and drop to change the order of WorkoutSet records, I didn't really care about re-ordering but it seemed like an easier place to start from than the full CRUD functionality.
 * In retrospect, I am not sure this was a good feature at all. But I wanted something relatively easy so that I could get more experience calling the django REST framework from javascript.
* It should be relatively easy now to add delete functionality to the page (arguably a much more useful feature than drag and drop).
* Big rework of the WorkoutSet REST API to support permissions checking and the django_measurement field - MeasurementField.
 * This got the endpoints working so that I could use them from the javascript side, along with maintaining a fairly uniform behavior between the REST framework and the controllers (so they are working with the WorkoutSet data using the same conventions).
The to_native and from_native are letting me do the juggling. The user should be getting and working with the user_weight_value (which is in their desired units), then before coming back into the database we convert it and hang on to weight_value. I would love to find a more clear solution than this, but from the serializer documentation this is the best I can come up with right now.