* guix-jupyter-container.scm: Add magic kernel command.
* guix-jupyter-kernel.scm: Add magic kernel command.
* guix-kernel/jupyter-server.scm: New file.
* Makefile.am: Add jupyter-server to SOURCE.
* guix-kernel/magic.scm (magic-kernel?,
  magic-get-kernel-name): Add procedure.
  (magic?): Change '%%' by ';;'.
* guix-kernel-demo.ipynb: Add new guix-kernel features.
* logo.png: New file.