demos: Use system monospace font stack following Style Guide
demos: Use system monospace font stack following Style Guide

Using a system orientated monospace font stack for best experience
(in)dependently of users operating system choice.
Also fixing UI sizing glitches in demo console resulting in one main
font-size chosen throughout demo interface.

Change-Id: Ied9021f97cce2f0e08965ac5dd956580cf8c6097

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui