Removed post.email from serialization when saving posts

closes https://github.com/TryGhost/Team/issues/1647

- sending the embedded `email` record back to the API when saving could trigger "Request entity too large" errors for very large posts that were sent as email because it doubles up on the request size
- `post.email` is a read-only property and is ignored by the API so there's no point serializing it and making request bodies larger than they need to be

ghost/admin/app/serializers/post.js

@@ -24,6 +24,7 @@ export default class PostSerializer extends ApplicationSerializer.extend(Embedde
         delete json.url;
         delete json.send_email_when_published;
         delete json.email_recipient_filter;
+        delete json.email;
         delete json.newsletter;
         // Deprecated property (replaced with data.authors)
         delete json.author;