Make draft chapters expandable

[nathaneastwood]

Hi, thanks for this great tool. I am using it for a book project of mine for a TCG called Flesh and Blood. You can see the project source here and the site here.

I have a lot of chapters. I am currently separating them with draft chapters with the following toggle:

[output.html.fold]
enable = true
level = 0

This enables the site to be navigated a little easier. Sections do not really work for me since I have so many chapters. The problem with the draft chapters is that they are a different colour (by default) and they are non-clickable, i.e. you cannot expand them - you must instead click on the arrow. Is it possible to make draft chapters clickable? It becomes a little counter intuitive when you can click on other sidebar links. This is possibly compounded by the fact I have changed the draft chapter colour to the same as the clickable chapters but I personally think this looks better.

[nathaneastwood]

I think this is very closely related to #483