Adds ctrl/cmd+s shortcut to whole application with a noop default

closes #4516
- shortcut for ctrl/cmd+s on application level
- shortens a syntax for any route that is using 'save' method, shortcuts object doesn't have to be defined at all, only the save action.
Would be good to have a comment about why we have an empty function in save.  Also, Ember has a standard for "placeholder function", inside of `Ember.K`. So you can do something like

```
// Turn unhandled save (triggered by keyboard shortcuts) into noop
save: Ember.K,
```