Only check status of important sensors

[nhoening]

Now we select all sensors of an asset (in addition to sensors on the flex context), but that might become too many for the status page to be useful (and fast enough).

Ideally, we'd be able to set asset sensors which are important on a field sensors_to_monitor or so. But we can also start with using sensors_to_show if it is set.

Let's make that the focus if this issue.

If we do only check a subset, we can add a small explanation at the top how we chose the sensors.