Update notification box styling, content

[JasonWeill]

References

Fixes #13992 and #13993.

Code changes

Updates the content of the "attempting to save read-only file" notification to match Jupyter styling with copy edits. Updates the CSS to put a small border around a notification display and to modify the padding for the header.

Adds another newline to the Jupyter news notification that appears on first startup, providing additional separation between the question and a clarifying request.

User-facing changes

Before the change:

After the change:

Backwards-incompatible changes

None.

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[krassowski]

The added border looks great!