Refactor textbooks to use locator URLs

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_textbooks.py

cms/djangoapps/contentstore/tests/utils.py

cms/djangoapps/contentstore/views/course.py

cms/static/coffee/spec/main_spec.coffee

cms/static/coffee/spec/models/textbook_spec.coffee

cms/static/coffee/spec/views/textbook_spec.coffee

cms/static/js/collections/textbook.js

cms/static/js/models/textbook.js

cms/templates/widgets/header.html

cms/urls.py