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>
restful: addition of validate method

* Adds a `validate` method in RestfulPagination class that checks
  the ranges of `page` and `per_page` parameters.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>