ANN: Added kernel logos to Cocalc-Jupyter in the kernel dropdown and selection panel

[williamstein]

In CoCalc's Jupyter, the kernel selection menu now shows the logos for the kernels, and the currently selected kernel in bold:

We also added logos and modernized the look of the kernel selection page that appears when you click the kernel status indicator in the upper right:

This looks a bit more like the JupyterLab launcher, though it's more organized, e.g., the different Sage kernels appear together: