Added mobiledoc.ghostVersion to blank mobiledoc document

refs https://github.com/TryGhost/Ghost/issues/12646

- when a blank mobiledoc document is created, ensure the correct Ghost version is added so content output is pinned to the version of Ghost that created it

core/server/lib/mobiledoc.js

@@ -13,6 +13,7 @@ module.exports = {
     get blankDocument() {
         return {
             version: '0.3.1',
+            ghostVersion: '4.0',
             markups: [],
             atoms: [],
             cards: [],