SD-661 : On comment save detect if any user has been mentioned on commen...

...t and generate the notification
At least at the first look at cerberus doc this is not possible (http://cerberus.readthedocs.org/en/latest/#schema-dict):
>>> schema = {'a_dict': {'type': 'dict', 'schema': {'address': {'type': 'string'}, 'city': {'type': 'string', 'required': True}}}}
>>> document = {'a_dict': {'address': 'my address', 'city': 'my town'}}
>>> v.validate(document, schema)
If the user can be embedded on list this is not required.
So this investigation will be done in other PR.