User Story 23, (EXTENSION 3/4) Book Index Page statistics shows top authors

```
As a Visitor,
When I visit the book index page,
In the Statistics area of the page,
I should also see three authors who have the highest-rated books
(average rating of all books for that author)
```

### Checklist:

- [ ] base code tests are written
- [ ] base code is written to pass tests
- [ ] edge case ("sad path") tests written
- [ ] edge case code written
- [ ] code is reviewed
- [ ] branch is merged
- [ ] all tests still pass

child of #5

child of #33

depends on #25