Remove <notebook>, use <kernel> throughout.

* guix-kernel/jupyter-client.scm (<notebook>, notebook): Remove.
(json->notebook): Rename to...
(json->kernel): ... this, and adjust accordingly.
(read-message, send-message, relay-message, pub, pub-busy)
(pub-idle, reply-html): Adjust accordingly.
* guix-kernel/jupyter-server.scm (<kernel>)[key]: New field.
(kernel-iopub): New macro.
(kernel): Add #:key.
(close-kernel): New procedure.
* guix-jupyter-kernel.scm: Adjust, replacing "notebook" with "kernel".
* guix-jupyter-container.scm: Likewise.