🐛 Fixed unnecessary "Are you sure you want to leave?" modals

no issue
- if a post was saved before the body content was edited you would get stuck with an "Are you sure you want to leave?" modal even though you had already saved
- the editor saw the post as being in a dirty state because the `mobiledoc` and `scratch` value were null but after saving the `mobiledoc` value was set to an blank mobiledoc object as returned by the API
- updated the `post` model save method to reset the `scratch` value if the returned `mobiledoc` no longer matches

ghost/admin/app/models/post.js

@@ -190,6 +190,19 @@ export default Model.extend(Comparable, ValidationEngine, {
         }
     }),
 
+    save() {
+        // ensure that our scratch value used in the editor is kept in sync with
+        // what we get back from the API after saving - avoids issues which cause
+        // the editor to see a saved post as dirty and show a "are you sure" modal
+        return this._super(...arguments).then(() => {
+            if (JSON.stringify(this.mobiledoc) !== JSON.stringify(this.scratch)) {
+                this.set('scratch', this.mobiledoc);
+            }
+
+            return this;
+        });
+    },
+
     _getPublishedAtBlogTZ() {
         let publishedAtUTC = this.publishedAtUTC;
         let publishedAtBlogDate = this.publishedAtBlogDate;