The kernel has access to the manifest, so it can display its actual
contents (including version numbers) upfront, before the environment is
built.

* guix/jupyter/inner-proxy.scm (kernel-info->shtml)
(kernel-info->html): Remove.
(reply-execute-request): Simply reply "Done!".
* guix-jupyter-kernel.scm (manifest->shtml)
(reply-for-environment, create-environment): New procedures.
(reply-execute-request): Call 'create-environment'.