Add back buttons to the mission framework

While I'm going through the mission, I keep wishing that I had a "back" button to go to the previous step. I forget what it said, and have had to start over a few times to see the previous step!

Permalinks would also be fine for this (as well as being useful to drop into irc when people are stuck and want help on a particular step).  As a workaround you could number the steps. 
