Mobile-friendly display

[hackmud-dtr]

On narrow screens, render this more like a stack of 3 cards (user list, channel list, chat area) so that it works on mobile.

( I'll try to work on this this week, just filing an ticket for it to remember )

[seanmakesgames]

#101 adds

@include breakpoint() sass mixin, which allows you to write simpler and more modular media queries (also ones that work on hackmud.com)

You can see some examples in the layout.scss file.