This replaces the old Extension.js_model_class attribute with a list

of JSExtension subclasses stored in Extension.js_extensions.
Each JSExtension subclass lists a JS model class. It also has an

apply_to field that works like those in bundles and TemplateHook. If not

provided, the extension will be initialized on all pages.
get_js_model_data has also moved to JSExtension, renamed to

get_model_data.
Extensions taking advantage of js_model_class in the Djblets 0.8 alphas

will need to be updated for this change.

Updated the rbseverity extension for this change, taking advantage of

apply_to, and saw the extension's initialization appear only on the

pages listed.