Supply usage statistics on/off flag in navapp appSettings

Make it possible for customers to disable sending out usage statistics by the navapp.

Do this by making use of the existing boolean JCR property send.usage.statistics.to.hippo  on the /hippo:configuration/hippo:frontend/settings node. Add a boolean named "usageStatisticsEnabled" to the appSettings JSON object so that the navapp can switch sending statistics on/off.