Add TOC collapse option to pressbooks-book (Luther) theme

When a book contains a large number of parts and chapters, the pop out table of contents becomes insanely long, to the point of not being usable. This PR adds a new theme option to the main Luther theme to enable TOC collapsing by-part. Enabling the option enqueues a small script that does the work.

As a sidenote, the click-to-expand/collapse behavior causes problems if the part has part text, so the script skips collapsing any parts that contain part text.

If you want to try it out, I have it live on [this book](http://reader.opentextbookstore.com/ushistory2/).
Am I missing something about how jQuery works?  I thought that since all the actions were chained off the selector, that it would only enter the DOM once. Doing a quick profile of the current code versus what you're suggesting doesn't seem to show a difference, unless I am misunderstanding what you were suggesting.