Optional skipping of email confirmation

* Add processing of `NEWSLETTER_CONFIRM_EMAIL...` in settings.
* Change `<Action>RequestView`s and `subscription_(un)subscribe.html` templates so that depending on settings:
  * user will be (un)subscribed right after submitting request form or an activation email will be send,
  * after submitting update request form user will be redirected immediately to update activation page or an activation email will be send.
* Add documentation for new settings.
* "Protect" existing tests from new settings.
* Add tests for subscription, unsubscription, update with activation emails switched off.

Related to #21.