Showcase archive: Set a default for the sorting filter

[ryelle]

When you land on the archive page, https://wordpress.org/showcase/archives/, the sites are shown in reverse chronological order (newest first), but the "Sort" filter does not have a value.

If you open it, you can select "Newest", and it does apply a filter– but it's the same state as the initial view. The filter should instead default to "Newest".