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().

Unit test passed.
Loaded an extension config form with a blank settings dictionary and

saw the defaults used, instead of blank fields.