Fix populating extension settings forms from defaults.

The move to SiteSettingsForm broke extension configuration pages
slightly. They accessed the settings object using settings.get(key), and
not through settings[key], which prevented any defaults from being
fetched.

There's now a get() function that works just like attribute access,
returning defaults, but with the semantics of the standard dict.get()
call. This complements set().

Reviewed at https://reviews.reviewboard.org/r/5805/