Fixed a routing error when a "sent" post was edited

refs https://github.com/TryGhost/Team/issues/954

- When an email-only post with a "sent" status was edited, it's status changed to "draft", which is an incorrect behavior. This change is causing the "sent" status to persist and removes the transition error.

ghost/admin/app/controllers/editor.js

@@ -470,6 +470,8 @@ export default Controller.extend({
                     status = 'published';
                 } else if (this.willSchedule && !this.get('post.isPublished')) {
                     status = 'scheduled';
+                } else if (this.get('post.isSent')) {
+                    status = 'sent';
                 } else {
                     status = 'draft';
                 }