refs https://github.com/TryGhost/Ghost/issues/9724
- fixed image card background colour
- made placeholder illustration for image cards work with Night Shift
- added viewbox prop to `summer.svg` illustration
- bumped ghost-spirit dependency
- fixed link discoverability when spellcheck is on
- fixed list spacing after headings 3-6 and refined list line heights
- added quick transition to format toolbar chiclet
- removed minimum width for image caption
- fixed wide image width in Safari
