NavigationBarHook.is_enabled_for only took a user as a parameter, which

meant that the hook couldn't easily differentiate between LocalSites

without subclassing.
Now, is_enabled_for is meant to take keyword arguments, and will be

passed the user, request, and the LocalSite name. Older implementations

that only take a user are still supported, but a warning will be shown.

Unit tests pass.
Used this in an extension that needed to know the LocalSite, and checked

that it worked both with and without the new function signatures.