Switch from pastanaga to volto-light-theme

[Fosten]

A key part of the next major plone.org release will be changing our default theme.
volto-light-theme provides many new features over pastanaga.

We should first consider upgrading Volto from 16.30.3 to 18.x.x so that we have the latest and greatest version.
And preparations have already begun with #180

But we should also be aware that various design elements may need customized to work with a new base theme.