Enhance Content Editor experience with drawer button to toggle state

The ability to edit directly in Customizer is AWESOME, but one more button would greatly improve its usability - a closing drawer button right above `the_content()` editor. I've prototyped this inside the browser here:

![screenshot from 2018-01-29 13-35-51](https://user-images.githubusercontent.com/963672/35535843-c7205d1e-04f9-11e8-9335-8d0f46801c8f.png)
