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/).
But the jQuery(this) is inside the click handler, so the "this" is referencing the specific item clicked, not the whole jQuery("#toc > ul") collection.  I'm pretty sure replacing jQuery(this).find with toc.find will end up toggling ALL the menu items, rather than just the one clicked.

Now, I do see that perhaps I should be caching jQuery(this).find(".dashicons") so it's not having to search twice, but considering that action only happens on a click, I'm not horribly worried about the efficiency.