refs https://github.com/TryGhost/Ghost/issues/9623
requires https://github.com/TryGhost/Ghost/pull/9666
- `{{koenig-card-embed}}`
- URL input
- perform oembed lookup & fetch on <kbd>Enter</kbd>
- remove card if enter pressed with empty URL
- show error message on server error
- "retry" returns to input bar
- "paste as link" removes card and outputs link
- "X" removes card
- force embedded <script> tags to run
- wrap embed html with `.koenig-embed-{video,photo,rich}` class
- add embed cards to the (+) and /-menus
- "section" support in the card menus
- refactor to use single card menu map and content component for both menus
- update /-menu keyboard movement to handle sections
- add parameter support to /-menu commands
- `/embed {url}` will insert embed card and automatically fetch oembed for supplied url
