Create mixin for common newsletter retrieving

Create NewsletterMixin with methods for retrieving newsletter for given
slug from given queryset and for adding newsletter to template context
and to form kwargs.

Change SubmissionViewBase to inherit from NewsletterMixin.
CBV for <action>_user views

Create base class ActionUserView inheriting from NewsletterMixin and
TemplateView.

Convert subscribe_user and unsubscribe_user to class based views
subclassing ActionUserView.
CBV for <action>_request views

Create base class ActionRequestView inheriting from NewsletterMixin and
FormView.

Convert subscribe_request, unsubscribe_request and update_request to
class based views subclassing ActionRequestView.

Adjust urls.py.
CBV for update_subscription view

Convert update_subscribtion to class based view inheriting from
NewsletterMixin and FormView.

Adjust urls.py.
Unify newsletter queryset retrieval

* Remove newsletter_queryset attribute from NewsletterMixin and it's
subclasses
* Make NewsletterMixin call NewsletterList's get_queryset to get
newsletter queryset
Remove old commented code

Remove whole get method from UpdateSubscriptionViev. the old commented
code was only thing that differed this method from parent method.
Add retrieving of not visible newsletter

* Change default newsletter queryset in NewsletterMixin to
Newsletter.on_site.all().
* Pass NewsletterListView().get_queryset() to
SubmissionViewBase.get_newsletter call.
Merge pull request #65 from jkuczm/cbv

Replacement of remaining function based views with class based equivalents.