Make Scroll within Panel inherit bottom rounded corners

[acelaya]

This PR addresses a slight visual glitch happening when rendering Panels without buttons, in which the bottom corners were rendered without roundiness.

Context

the Panel component is a wrapper around Card, which conditionally renders <CardContent /> as the last children if some buttons have been provided, and can also wrap the panel content with <Scroll /> if scrollable prop is true.

For simplicity, the default value for scrollable is true, so that you don't need to remember adding it and it just works if the content is too long.

During the development of hypothesis/client#5938, we found some Panels and Dialogs where bottom corners were not rendered as rounded, because they didn't have buttons. It was worked aroundt by setting scrollable={false}, as this was not needed: hypothesis/client#5938 (comment)

With these changes, that workaround would not be needed anymore.

Testing steps

The pattern library included two similar examples of scrollable panels. I have edited one of them so that it does not include buttons, in order to verify rounded corners work.

1. Check out this branch and go to http://localhost:4001/layout-panel.
2. Use the browser's search capabilities to search for "scrollable".
3. Check both examples render bottom corners as rounded.