Fixes the issue introduced by commit 7cb8dcc which removed the
_external-link.html partial. The external link logic is now handled
inline in the card.html template to properly support both internal
and external URLs.
This greatly reduces the number of inline styles in the project and enhances maintainability by avoiding a lot of duplicate code.
This also reduces the number of hashes required for a CSP since all previous background-image inline styles are now in main.css.