Increase padding or margin on frontmatter block

[stevejpurves]

The frontmatter block currently renders right up to the edge of the cell and could benefit from additional padding (and potentially a max-width property being set for wide notebooks) overall, however indepdnent of available wide the top of the frontmatter block overlaps with the jupyterlab pup up cell toolbar, as shown:

Additional padding / margin could address this.