Fix URL pointer to Help page for sensor components

Change-Id: I658540a00dc760c514a080071c7bf5128bce50e9
Only permit one session per user

Only permit one session per user. Newer sessions cause older sessions
to become read-only. Enforcement is controlled by a flag in
AppInventorFeatures.

Change-Id: Ie493b6477473dc8d9377690a044a9fa012e2f27c