Use template to generate TOC

[jacwah]

Instead of directly writing html from Rust, use a Handlebars template to generate the table of contents. This requires build a separate json object from chapters. To remove the duplicated data, the second commit adapts navigation to use the new toc json object instead, and is an alternative to #465.

Pros:

• toc is easy to edit and customisable
• code is more maintainable

Cons:

• the template generates funky html with lots of whitespace (will look for a solution)
• complicated template -- some handlebars knowledge required
• book chapters are processed in two steps (can probably join them to cut down code)

I'm not quite sure how to organise the code. Perhaps toc_json.rs could be merged with helpers/toc.rs. I'm also not sure when to trust the code and use unwrap and when passing along errors is preferred.

#458