sessions: use 'user_id' value as fallback value for the user id
sessions: use 'user_id' value as fallback value for the user id

* sometimes, the value '_user_id' wouldn't be stored in the session, but
  'user_id' would be

inveniosoftware/invenio-accounts,inveniosoftware/invenio-accounts,inveniosoftware/invenio-accounts