Unify `action in ...` assertions

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

* Add `ProcessUrlDataMixin` that processes args and kwargs from url
  before before dispatching request.
  
  Before processing of url data was done in `dispatch` method.
  Since value returned by `dispatch` is crucial it should always return
  result of calling `dispatch` from superclass.
  With such setup view instance attributes assigned by dispatch method
  of superclass could not be accessed while assigning attributes of
  subclass, but since this access was needed all assignments were done
  only in final subclasses, not very DRY.
  
  Solved by adding `process_url_data method` that is also called in
  `dispatch`, but now in `process_url_data` in subclass, superclass's
  `process_url_data` can be called in any place (or even not at all).
  
* Adapt `NewsletterMixin`, `ActionUserView`, `ActionRequestView`,
  `UpdateSubscriptionViev` and `SubmissionViewBase` to inheritance from
  `ProcessUrlDataMixin`.

* Add `ActionMixin` mixin retrieving action and adding it to context.

* Adapt `ActionUserView`, `ActionRequestView`
  and `UpdateSubscriptionViev` to inheritance from `ActionMixin`.