proposed changes to the code review lesson

I have some ideas for improving `novice/extras/02-review.md` and would like some feedback before I start working. I propose the following:

1. split out a lesson on creating and maintaining your own fork, to be taught before the code review lesson
2. enhance the existing code review lesson to teach how to create pull requests between branches within one repo and across forks
3. flush out a little bit of technique for how to review code

I'm not sure if point 1 needs to be a separate lesson or if this can be structured together as one. Point 2 supports a simplified branching model I see at some companies and expect to see at some labs where individuals all work on branches of the parent repo.
Possible challenge: Fork the planets repo, clone the fork, make a change and submit a PR.