Request Download

This PR adds a "request download" button to the layer and document detail pages when the authenticated user does not have the download_resourcebase permission for that resource (as per https://github.com/GeoNode/geonode/issues/255).
The notification will be sent by django-notification to the resource owner.
A status message element is added to the user interface of GeoNode, and it can be used also for other messages/alerts to the user. In this context the user is notified with a message in the status message element if the request was sent or not to the resource owner.