make bokeh.resources.JSResources and CSSResources crawl only models in document for external resources

[canavandl]

Currently when a new ViewModel is defined (in bokeh/model.py) it's added to a model_class_reverse_map. When Bokeh collects the necessary resources with the JSResources and CSSResources objects (in bokeh/resources.py), it crawls the model_class_reverse_map dict and checks each model for a __javascript__ and __css__ attribute.

The issue is that there are use cases where a user will create custom models that aren't included in the document. Those models' resources will be included in the output element regardless. Apart from slower page loads, it's possible to have conflicting resources.

The solution is likely to make sure that the model is actually in the document.