MW UI: Show all permutations of checkboxes in Living Style Guide

* Regular unchecked
* Regular checked (added)
* Disabled unchecked
* Disabled checked

Although you can obviously check and uncheck it yourself, it's easier
for the user to show them the permutations on page load.  Otherwise,
they might not even notice the special check mark image.

Also, fix a couple unrelated typos in buttons.less.

Change-Id: I17b63eeb424e23d1c3b9ce9ab8bfaa66b5bf4a46