CLC-3783 Textbooks may include materials without an ISBN

https://jira.ets.berkeley.edu/jira/browse/CLC-3783

As you can see from the commit list, this includes many other changes as well.

* Finally support non-testext non-production-server textbooks tests.
* Improve existing tests & reactivate the choice-of-materials test.
* Add a dummy JSON for more convenient client-side development.
* Reinstate targeted links to the bookstore. (These were originally tried in https://github.com/ets-berkeley-edu/calcentral/pull/1982 but then reverted because direct links weren't loading correctly. They now work in my testing, & so we'll give them another try.)
* Changed the layout somewhat to take advantage of bookstore links, including an icon for the Cal Student Store.
VCR's also deprecated and we don't want to introduce new instances of it. I like the new slim-fixture approach seen here.