Use `box-sizing: border-box` everywhere
Apply left and right padding to app container

Use spacing variables from [SUIT example theme]
(https://github.com/suitcss/theme/blob/master/lib/theme.css) for
this.
Use spacing variable for grid gutter size

Also rearrange styles so more general ones are on top, more specific
ones below.
Tweak spacing between staff and guess entry

Reduce staff SVG height to smallest amount necessary. Then use CSS
variable to add aesthetic space consistent with rest of app.
Add spacing above staff
Move Staff component JS and CSS into own directory
Merge branch 'style-improvements'