Open first module with book

This implements a bit of API, a model can direct the editing machinery to edit something else, some representative content instead. This is then applied to the opf-file model, so that opening a book edits the first module in the book.

Outstanding:
- [x] 3. Using the same window in the browser, change the repo on the URL. (The first module doesn't open).  
- [x] 4. Select change repo in the gear thing. First module of first book should open. 
- [x] 5. Open a repo on a branch. (Should do the same as 1.)
- [ ] 6. Fork a repo. Should stay exactly where you were - same book open, same module open.  (Definitely have stories for this case)