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.
@raydavis the `Cal Student Store icon`. We shouldn't be adding `png` icons / logos.