Polish sites showcase

[thibaudcolas]

Polish sites showcase #105 is on the Wagtail roadmap. We have yet to confirm exactly what the polishing will consist of but here are the ideas that have been discussed so far:

• More content for the Showcase page, in particular from all around the world, different kinds of sites
• A design that supports displaying more items
• More filtering potentially?

[vossisboss]

We need to improve the navigator at the bottom and how the CTA is displayed too.