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.
*Moved comment from commit to PR*

Whoops, there should probably be an if similar to [this one](https://github.com/Gargol/Ghost/blob/issue-4516/core/client/mixins/shortcuts-route.js#L80-L84) inside of the deactivate hook, rather than adding an explicit shortcuts object.

Originally, I was thinking we'd throw an error if someone mixed in the shortcutsroute mixin without actually adding any shortcuts, hence the lack of expressly adding one.

...... But if that's not the option we want to go, then maybe you're right in adding in a default hash. If you want to do that, go ahead and delete the if out of the activate hook, eh?