Add sponsor logos to the public certificate page

Sponsors put money into the event. Their logo usually appears on the certificate artwork and nowhere else.

Sponsor slots are now available on the public certificate page. Add sponsor logos with links and short captions in a dedicated area, separate from the award details. Because the sponsor area belongs to the page, it can change without republishing the certificate design.

A printed logo is static. A sponsor slot can link to the sponsor's site and stay current after the event, which makes the sponsorship more useful to the sponsor and easier for the organiser to fulfil. Sponsor slots are a Pro feature; the certificate itself remains the recipient's record.