reorder git setup into the git lessons

[mbjones]

In the past, we used to introduce the concepts behind github before we set up the tools. During today's arctic lesson, we set up the complex git setup before explaining any of it. We should rearrange the git setup to be included in the git intro lesson itself, which will allow it to follow some of the intro material. It will also allow us to shorten the intro setup section to get into the more interesting RMarkdown sooner.