This adds the ability to specify one or more default bundles to load
when calling the load_extensions_css and load_extensions_js template
tags. Each now take a third parameter, specifying a comma-separated list
of It adds a third parameter bundle IDs to load, defaulting to
"default" as before.

By allowing the default to be changed, pages can load certain
specially-defined bundle IDs, like "tests".

Made use of these in an upcoming Review Board change.