Remove remaining jQuery

This avoids every page on the site parsing 250KB of JavaScript.

supporter-page.js isn't the most elegant code, but at least it's a little more
modular now and no longer user jQuery.

Replace complex expanding JS with native <details> elements

Have retained the "expand all" behaviour - just with less code.

Remove nesting of hidden sections on sustainer page

This makes the page more similar to how it has worked in previous years.

Fix "expand all sections" second link

Issue was that the jQuery is using .children() to add the event handlers but due
to some changes to the HTML, the second link is no longer a direct child. I've
switched this to .find() instead.