Mobile Responsive Styles (#580)

* WIP: Responsive styles

* Flip breakpoints, ensure doc doesn't spread

* Add MenuIcon

* Refactor Sidebar to share mobile responsive styles

* Fix accidental find/replace

* Tweak padding to take into account icon spacing

app/components/Editor/components/Image.js

@@ -94,6 +94,7 @@ const Caption = styled.input`
   text-align: center;
   width: 100%;
   outline: none;
+  background: none;
 
   &::placeholder {
     color: ${color.slate};