Replace "Interface" dropdown with button if only one option is present

[JasonWeill]

The "Interface" dropdown in JupyterLab is a large dropdown in the toolbar with only one option, "Open With Jupyter Notebook":

… and in Jupyter Notebook, it's a dropdown with one option, "Open With JupyterLab":

If only one option is present, replace this dropdown with a single button with an icon for the other interface, also on the toolbar, to save space.

In addition, add a command to the right-click context menu on the notebook body to "Open With" the other interface.

This was found during a Bug Bash by @ellisonbg .

[jtpio]

For reference there used be buttons in the toolbar before (in RetroLab): jupyterlab/retrolab#256

Which was then switched to a menu to avoid confusion: jupyterlab/retrolab#309

There is also #6746

[JasonWeill]

Thanks @jtpio! From jupyterlab/retrolab#309, here's the "before" picture showing RetroLab with buttons for "open in JupyterLab" (Jupyter logo with 3 moons) and "open in classic notebook" (Jupyter logo with no moons).

[JasonWeill]

After discussion with @ellisonbg , this requested change is revised to:

1. If the interface menu has only one option, display a button using a UI as demonstrated in Changes interface menu to button #6793.
2. If the interface menu has more than one option, display a menu with multiple choices.

[jtpio]

this requested change is revised to:

Will that change actually be needed if #6746 is implemented?

If notebook depends on both nbclassic and jupyterlab, there will always be at least two entries.