Add basic "downloadable" support for ExportMasterView

instead of it trying to do its own thing for that...  more to come on this

tailbone/views/master.py

@@ -2767,6 +2767,13 @@ class MasterView(View):
         """
         return getattr(cls, 'mobile_row_form_factory', forms.Form)
 
+    def download_path(self, obj, filename):
+        """
+        Should return absolute path on disk, for the given object and filename.
+        Result will be used to return a file response to client.
+        """
+        raise NotImplementedError
+
     def render_downloadable_file(self, obj, field):
         filename = getattr(obj, field)
         if not filename: