Upgrade sensors_to_show to its own data model attribute

[nhoening]

GenericAsset.attributes should be for dev or custom features, and sensors_to_show has become an important feature.

Editing it among all the others is also cumbersome.

So we should move it to its own place, GenericAsset.sensors_to_show.

Note that there is already a property for sensors_to_show on Genericasset. That should keep on working, its contents need refactoring.

Maybe we can discuss if the data migration code can also migrate this data from attributes. Probably we should do that with some Python scripting.