common (template): better json dumping:

default method is set for json dumping (the `tojson` filter), if the object has a `json`
attribute, it will be called, otherwise it is replaced by None, this avoids crash when
rendering the template.

sat/tools/common/template.py

@@ -451,6 +451,11 @@ class Renderer(object):
 
         # policies
         self.env.policies["ext.i18n.trimmed"] = True
+        self.env.policies["json.dumps_kwargs"] = {
+            "sort_keys": True,
+            # if object can't be serialised, we use None
+            "default": lambda o: o.json() if hasattr(o, "json") else None
+        }
 
     def getFrontURL(self, template_data, path=None):
         """Give front URL (i.e. URL seen by end-user) of a path