Update dependency django-guardian to v2

requirements.txt

@@ -9,7 +9,7 @@ click==7.1.2                # via pip-tools
 decorator==4.4.2          # via ipython, traitlets
 django-cors-headers==2.5.3
 django-extensions==2.2.9
-django-guardian==1.5.1
+django-guardian==2.4.0
 django-webpack-loader==0.7.0
 django==2.2.28
 djangorestframework==3.15.1  # via open-widget-framework