User Story 7, Book Index Page shows Average Rating

[iandouglas]

As a Visitor,
When I visit the book index page,
Next to each book title, I see its average book rating
And I also see the total number of reviews for the book.

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 #26

child of #33

depends on #25