support for NoRack and NoDC attributes from viewer/nodes handler

[adameat]

whenever backend knows that there are no DCs or Racks it will signal to UI to hide corresponding columns.
it would be better to automatically hide this columns upon receiving such flag.

[Raubzeug]

For now we have a control that allows user to select visible columns. I think, if user explicitly selected Rack or DC column to be shown, we should show it anyway.
By default Rack and DC are hidden.

@adameat do you see any ways to use NoRack and NoDC attributes?