settings_manager.init(): Define the initial values from the user_settings spec.

spideroak_init(): Call settings_manager.init().
settings_manager.get_immediate(): Get the most recently obtained value for
a setting, and weave the storage of that value into the getter.

settings_manager.define(): Do an initial .get() of default values, so
settings_manager.get_immediate() is properly primed.
apply_settings(): apply_settings_values() still incomplete, but the promise
conclusion (both resolve and reject) is adequately filled in.

Incidental note: obscure jQuery bug in deriving more refined
selections. Eg:

        $page.find('[so-setting-display]')
             .find('[so-setting-display="content"]')

    doesn't work, because the resulting selector is:

        "... [so-setting-display] [so-setting-display="content"]"

    I think incidence of has-attribute and more specific attribute
    selectors on the same attribute should be unified to the more specific
    selectors.