Add NEWSLETTER_CONFIRM_EMAIL... settings

Add processing of NEWSLETTER_CONFIRM_EMAIL setting and
NEWSLETTER_CONFIRM_EMAIL_<ACTION> settings. Result is stored in
CONFIRM_EMAIL_ACTION dictionary and can be used to decide whether
confirmation email is needed for particular action (subscribe,
unsubscribe, update).
Optional skipping of email confirmation in views

Depending on setting stored in CONFIRM_EMAIL_ACTION,
<Action>RequestViews will subscribe, update and unsubscribe user without
email confirmation.
Adjust templates to skipping email confirmation
Document NEWSLETTER_CONFIRM_EMAIL... settings