New Feature: add a pop-out quick ref

[funkybob]

Issue description

Feature suggestion: pop-out quick reference in tutorial

At the DjangoGirls event I mentored this weekend, I found a lot of participants flicking back frequently to find how to do commands they'd done before, most often how to re-activate their virtualenv.

If there were a pop-out sidebar which contained some of these frequently needed commands, it could save a lot of lost positions.

A stretch goal might be to have it grow as you go through the tutorial, but that may be excessively burdensome to maintain.