Larger screens: "Reverse pyramid" button layout

Set guess-entry button size to 1-of-4 instead of 1-of-7 on larger
viewports. Also center align them. Together these two changes create
the "reverse pyramid" layout for larger viewports.
Smaller screens: Stacked guess-entry buttons
Smaller screens: Remove space between buttons

Change `Grid--withVerticalGutter` to `Grid--md-withVerticalGutter` and
make it only apply on larger viewports.
Rounder corners on buttons

Rebecca prefers this `border-radius`.
Reduce height on guess-entry buttons

Rebecca prefers this height.
Merge branch 'responsive-guess-entry-button-layout'