Use a `PluginRegistry` for loading plugins

[jtpio]

Current the NotebookApp does not make use of a PluginRegistry for loading plugins:

notebook/app/index.template.js

Line 219 in ab63d5c

app.registerPluginModules(mods);

It also does not resolve ServiceManagerPlugins first as in JupyterLab.

Notebook should follow the same approach as in JupyterLab: https://github.com/jupyterlab/jupyterlab/blob/eaddbc7b6e8a13198edd9cdb156def78724f07e2/dev_mode/index.js#L260

As a follow-up to jupyterlab/jupyterlab#16794.