Add "activation email sent" pages

* Add `ActivationEmailSentView`.
* Add `<newsletter_slug>/<action>/email-sent/` urls.
* Add `subscription_<action>_email_sent.html` templates.
* Add tests for new view.
Unify `action in ...` assertions

* Add `ACTIONS` tuple to utils.py.
* Use `utils.ACTIONS` in views.py, models.py, test_maling.py.