Add new theme options to Candela theme

Is does the same thing as my earlier pull request, but adds it to the Candela theme rather than the opentextbook theme.  Here's the details again:

This is a set of enhancements to consider at some point. They add three new theme options to the Candela 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