Easier editor entry

Ref #2308

- Double clicking a PostItemView on the content screen will open that post
  in the editor.
- Added `'ctrl+e, command+e': 'openEditor'` shortcut will open editor as well. Added `openEditor` action to `PostsPostController` to this end.
@rjackson This code ends up more-or-less duplicating the openEditor action of the `Posts` route. I'd love to just let the `PostsRoute` do all the work, but the PostsPost route needs to add its model to the action before the action is complete.
Is it possible to handle an action, and then bubble it up with a parameter attached?

something like
```javascript
//PostsPostRoute
sendOpenEditor: function() {
    this.send('openEditor`, this.get('controller.model'));
}
```
And since `this` wouldn't have openEditor defined, the newly sent action would get handled by `PostsRoute`?