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.
You've just used fork and clone quite close to each other, and I think learners might be confused about the difference. Maybe an explicit statement to the effect that if Wolfman clones Dracula's repository to his desktop, he won't be able to push changes to it (unless Dracula gives him permission), but forking makes a new repo owned by Wolfman.