Fork improvements

This implements one thing: If you press the fork button while logged in, it will prompt you to log in, and continue with the fork afterwards. It's only step one. What is needed:

- [x] You are not logged in. -- Needs to ask you to login in to the account you want to fork the book into. 

- [ ] You already have a fork of this book. -- Needs to say "You already have a copy of this bookshelf. [Take me there] [Cancel]."

- [ ] Expected case: You are logged in and you don't have a fork yet. -- Needs to ask you to supply an id for this bookshelf and supply the current id by default. 

- [ ] "Please provide an ID for your copy." [current-id-by-default] Valid ids? - Hover says "You can use letters, numbers, and hyphens. Don't use spaces.") 

- [ ] Once the fork has occurred, need to poll to see when it is complete, before returning the author to the repo. This change should probably go into octokit. 

- [ ] If possible, the changes that the author made before the fork should be saved back to the new forked copy. If this isn't possible, before doing the fork, need to warn that changes will be lost.