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>
> to check if there are some rows with `''` value? (and convert it as `'P'`?)

Does `''` have same meaning like `'P'`? If not then maybe it's enough just amend the `api` with default argument value.