Revert "Implemented header collapsing." (#191)

This reverts commit f81dc85aa4e9d31020d448a1918a550b2e0c4ee4, due to a
buggy implementation of header collapsing. Major issues:

* Hides footnotes if the last section is being hidden (and shows the
  quiz buttons on non-quiz pages)
* h4/h4/h5/h6 headings are ignored
* Clicking the edit/¶ links triggers the toggle

Will be reimplemented once I figure out a clean way of determining sections
dynamically, most likely with a link next to "edit".