For server-owned interfaces, display a form to send data to the device

[davidebriani]

Since server-owned interfaces are designed for handling data published via the Appengine API instead of published by devices, it can be valuable to improve the page that displays data for the interface and add a form that allows to send data.

The form should respect the endpoints and data validation as defined by the interface, providing a handy means to publish data.

For this feature, the issue #416 can also be relevant: the form could ideally be hidden or disable if the user doesn't possess sufficient authorization to publish data on that interface, or for that device.