Notebook modal dialog is not properly represented as a dialog

[acelaya]

Part of #6151

To make the dialog more accessible:

1. Add a role=”dialog” to the dialog container. This will allow assistive technologies to announce the dialog as it appears.
2. Add an aria-modal=”true” attribute to the dialog. This will hide background content from assistive technologies while the modal is open.
3. Shift keyboard focus into the dialog when it opens, and constrain focus within the modal until the user closes the dialog.