Bring back CLI instructions for checking out PRs #963

I'm talking about Step 1 in the section that comes up if you click 'view command line instructions' at the bottom of PRs. I just moved to a new laptop where I don't have hub installed yet and had to discover that checking out PRs without it is surprisingly cumbersome 😅

Using this box, you just have to click the copy button and paste it into the terminal to check out a PR locally. Maybe we can bring it back somehow, perhaps just have the copy button at the top of PRs without showing the commands.