User Story 22, (EXTENSION 2/4) Review usernames are unique for a given book

[BrianZanti]

As a Visitor,
I should not be permitted to save a book review using a name
I've previously used to review a specific book.

(user names are unique for a specific book's reviews)

DO THESE STORIES FIRST:

• User Story 13, Add a Review for a 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 #5

child of #33

depends on #16