closes: https://github.com/TryGhost/Ghost/issues/8179
- previously all cards just had a `__mobiledoc-card` class on them, this update replaces them with a class `kg-card` and adds an additional class of `kg-{{card_name}}`
refs https://github.com/TryGhost/Ghost/issues/8106, https://github.com/TryGhost/Ghost/issues/7429, requires https://github.com/TryGhost/Ghost/pull/8137
-Adds new "card" menus
- Navigation with keyboard in both axis.
- Search with keyboard in both menus.
- Adds a "+" Menu for cards
- Adds a "/" Menu for cards
- if the block has content and it becomes a markdown or HTML Embed card then the content is included into the card.
- Image and HR cards appear below the current section
- Adds new toolbar with both inline and block styling.
- Adds a new 'divider' card.
