update banners, and one of the project links
update links to point to main challenge projects (per Chris)
Merge pull request #1304 from jay-hodgson/SWC-1882

Swc-1882: update challenge links and images
add new link to front page for Collaboratorium
Merge pull request #1306 from jay-hodgson/release-66

SWC-1895: add new link to front page for Collaboratorium
Merge branch 'release-66' into develop