Generate default groups on the fly

Current error:

Internal Server Error: /explorers/user/createExplorer/
...
/backend/SAF_django/Explorers/utils.py", line 18, in newExplorerProcedure
    group = Group.objects.get(name=EXPLORERS_GROUP_NAME)
...
    raise self.model.DoesNotExist(
django.contrib.auth.models.Group.DoesNotExist: Group matching query does not exist.

Should work for

• Carto groups
• Explo group