Jupyter notebook branding

[embray]

I'm working on branding-related issues for the installer and desktop icons and the like.

One open issue is that when the jupyter notebook launches it has the top left "jupyter" logo image, which might be confusing to a new user expecting to get "Sage". It's true that when they start a notebook using the SageMath kernel, the SageMath logo is displayed in the top right.

It shouldn't be too hard to replace the jupyter logo, but need to make sure this doesn't violate any usage terms (I don't think it would but good to check).