Initialize handler page_config from Server traitlets

[bollwyvl]

This initializes page_config from, e.g. ServerApp.tornado_settings.page_config_data. These values can be subsequently overwritten by other configurables.

This corresponds to LabHandler in jupyterlab_server.

[github-actions]

👈 Launch a Binder on branch bollwyvl/notebook/add-default-page-config

[jtpio]

Thanks!

Looks like this adds the following two items to the File menu (noticed by the UI tests):

Which is fine. We can always re-work the snapshots later via the settings system so they look more like the ones from the classic notebook:

[jtpio]

Thanks!

[bollwyvl]

Thanks!