Koenig - Versioned renderer (#9606)

refs TryGhost/Ghost#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

packages/kg-default-atoms/lib/atoms/index.js

@@ -1,3 +1,5 @@
-var softReturn = require('./soft-return');
+'use strict';
+
+const softReturn = require('./soft-return');
 
 module.exports = [softReturn];

packages/kg-default-atoms/lib/atoms/soft-return.js

@@ -1,3 +1,5 @@
+'use strict';
+
 module.exports = {
     name: 'soft-return',
     type: 'dom',