new_contents: from feedly.activity import Activity
  from feedly.storage.cassandra.maps import ActivityMap
  from feedly.storage.utils.serializers.base import BaseActivitySerializer
  from feedly.verbs import get_verb_by_id
  import pickle
  
  
  class ActivitySerializer(BaseActivitySerializer):
  
      def get_serialized_activity(self, activity):
          return ActivityMap(
              key=activity.serialization_id,
              actor=activity.actor_id,
              time=activity.time,
              verb=activity.verb.id,
              object=activity.object_id,
              target=activity.target_id,
              entity_id=activity.extra_context.get('entity_id'),
              extra_context=pickle.dumps(activity.extra_context)
          )
  
      def loads(self, serialized_activities):
          activities = []
          for serialised_activity in serialized_activities:
              activity_kwargs = serialised_activity.__dict__.copy()
              activity_kwargs.pop('key')
              activity_kwargs.pop('entity_id')
              activity_kwargs['verb'] = get_verb_by_id(activity_kwargs['verb'])
              activity_kwargs['extra_context'] = pickle.loads(
                  activity_kwargs['extra_context'])
              activities.append(Activity(**activity_kwargs))
          return activities
  
subject: add cassandra activity serializer