Update the logos for the demo links

[jtpio]

At the moment the JupyterLab shows the Jupyter logo, and the Notebook demo shows the Python logo.

This was briefly mentioned at the weekly JupyterLab meeting last week (jupyterlab/team-compass#135 (comment))

Maybe we should update the logos to the following?

JupyterLab

This one doesn't seem to be official, but is used for the GitHub organization:

or

Jupyter Notebook

or

Here is what it could look like:

cc @afshin who brought that point

[afshin]

I think your proposal is an improvement. I wonder if the Jupyter Notebook logo should say notebook in it. What do you -- and others -- think?