Use api to populate widget models

Currently, we keep a client config dictionary on the server side for each widget, and pass this over to populate the client side models. We currently have an api method for accessing each widget's details. Getting rid of the client config stuff we are doing all over the widgets and instead using this api method to populate the client side models will clean up quite a few things, and is probably a necessary step for our move over to rendering dashboards client side (#67).