Class based views

Ported all remaining function views to class based, DRYing them as far as I could.

* Retrieving of newsletter moved to `NewsletterMixin`
* common parts of `subscribe_user` and `unsubscribe_user` moved to `ActionUserView`
* common parts of `subscribe_request`,  `unsubscribe_request` and `update_request` moved to `ActionRequestView`