Add NEWSLETTER_CONFIRM_FORM... settings

Add processing of NEWSLETTER_CONFIRM_FORM setting and
NEWSLETTER_CONFIRM_FORM_<ACTION> settings. Result is stored in
CONFIRM_FORM_ACTION dictionary and can be used to decide whether
activation or particular action (subscribe, unsubscribe, update) needs
to be confirmed by submitting activation form.

Update receiver of setting_changed signal to cover new settings.