accounts: enhancement in UserUsergroup

* Adds in the model the update of the date after last change of
  user status.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
Probably the best is to convert it in a hybrid_property and put inside it the permission controls: https://github.com/hachreak/invenio/blob/enhance_account_model/invenio/modules/accounts/models.py#L191