Add a CSV export functionality in admin and add price fields (#41)

* Implement an action do download data in csv
* Refactor CSV download
* Move price to main models and add csv to bookshelf
* Update template and API
* Small refactoring

ram/roster/serializers.py

@@ -28,5 +28,5 @@ class RollingStockSerializer(serializers.ModelSerializer):
 
     class Meta:
         model = RollingStock
-        fields = "__all__"
+        exclude = ("price",)
         read_only_fields = ("creation_time", "updated_time")