Don't show the notebook variable view when not applicable

[amunger]

We should at least keep the view collapsed if a notebook controller isn't able to provide variables.
Also hide/disable the view if the setting is disabled at any point.

[DonJayamanne]

Unfortunately I cannot get the variables view to show up.

[DonJayamanne]

Install the github issues notebook extension

• OPen ipynb file and Notebook Variables view is displayed
• Open a gibuthubissues notebook and ensure the other ipynb is not visible
  I would expect the Notebook Variablers view to be hidden.

[amunger]

I think we'll end up needing to show the variable more often for any open notebook and just showing the notebook variable source instead.