Theme options for TOC, source-specific styles, MathJax

This is a set of enhancements to consider at some point.  They add three new theme options to the opentextbook theme:

1) An option to make the pop-out table-of-contents expandable/collapsible by part.  Clicking a part title will toggle it open/closed. By default, all parts are closed except for the part containing the current chapter, and any parts with Part Text. This will help make courses with a ton of chapters more manageable to navigate. Also adds an indent after the first line when a chapter title wraps to help differentiate a wrapped line from a new chapter.

2) An option to load source-specific styles, to deal with some display issues with imported content without having to resort to inline styles.  So far I have added two:
a) OpenStax: Boxes exercises, and adds JS to make exercise solutions toggle to show. Insets figures and a few other small tweaks.
b) Lardbucket: uses styles identical to the editor bcc-box styles to box some key features. Insets figures and a few other small tweaks.

3) An option to enable MathJax for the book, loaded from the CDN using the most general config: TeX-MML-AM_HTMLorMML

(sorry the candela license fix snuck into this pull request too - I didn't separate my branches properly)