Pass extra data to the usage statistics script for measuring NPS

The collection of usage statistics starts when the CMS published the event 'start-usage-statistics'. Currently only the events queued so far are passed along with this event. To measure the NPS score we also need to pass a token that identifies a CMS user.

The idea is to implement this token as a hash based on the user id (e.g. 'admin') and the hostname of the CMS (e.g. cms.onehippo.org). Together that should result in a unique ID for a CMS user without revealing actual user IDs or hostnames.