Review fixes for GenericDigitalSignViewMixin
Review fixes for GenericDigitalSignViewMixin

- Bugfix: Use DIGIDOC_SESSION_DATA_KEY instead of DIGIDOC_SESSION_KEY
  in destroy_digidoc_session_data.
- Ensure hanging references to digidoc session data are cleared when
  digidoc session data is destroyed
- Use .get instead of catching KeyError inside get_digidoc_session

thorgate/django-esteid,thorgate/django-esteid,thorgate/django-esteid