Fix hidden prompt overlay button

[gnestor]

Closes #2590

It looks like this.collapsed is returning false and therefore this.prompt_overlay.show() is not being called: https://github.com/jupyter/notebook/blob/master/notebook/static/notebook/js/outputarea.js#L155

[gnestor]

@minrk Mind taking a look at this? Just wanna make sure that this doesn't behave unexpectedly in some edge case.

[Carreau]

Let's try it to see if it has side effects.

[minrk]

I think this is right. I saw this when I did my PR, but then forgot to investigate.

[jasongrout]

FYI, this breaks the prompt_area: false argument to the output area constructor. We're seeing this in ipywidgets, where we explicitly set that to false, but now the prompt area is still shown.

For now, we'll just hardcode some css to hide the prompt area in output widgets. I think it would be enough to check this.prompt_area and if it is false, explicitly hide the prompt area as before.

CC jupyter-widgets/ipywidgets#1765

[jasongrout]

Proposed fix: #2961