This introduces Extension.get_static_url(), which takes a static media

file bundled with the extension and returns the URL for it. This is

equivalent to what the ext_static template tag does, and in fact that

template tag has been updated to use this. It's much easier now to

reference extension-provided static media files outside of templates.

Tested that templates were still able to reference the static files.
Made use of the new function in some of the new integrations work.