refs https://github.com/TryGhost/Ghost/issues/9505
- updates mobiledoc converter's `render` method to accept a `version` argument
- `1` === Ghost 1.0's markdown-only renderer output
- `2` === Koenig's full mobiledoc renderer output
- switch between mobiledoc renderer versions in Post model's `onSaving` hook
- version 1 by default
- version 2 if Koenig is enabled (currently behind dev experiments config + labs flag)
- version 2 if the post's mobiledoc is not compatible with the markdown-only renderer
- "version 2" full-Koenig mobiledoc renderer output
- wraps content in a `.kg-post` div
- removes wrapper around markdown and html card output
- adds classes to image card output including selected image size/style
- standardises es6 usage across mobiledoc related files
