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>
it was set a default `''`, but it is not a consistent state because it has to have one of three possible values. So, normally, after the creation of a `UserUsergroup`, it's always set the user_status value.