This is a proof-of-concept for the proposal #8021

Work-in-progress, not intended to be merged

To use:

• Once GeoNetwork is running, go to System Settings and enable the Datahub; give it a custom configuration if needed
  The Datahub can be accessed on http://localhost:8080/geonetwork/srv/datahub
• The Datahub app can be also be enabled for subportals; a different configuration can be provided
  In that case it can be accessed on http://localhost:8080/geonetwork/{subportal}/datahub