Add side floating navigation to the content (about) pages

This PR implements the design for https://bugs.caret.cam.ac.uk/browse/PAVO-111. A side navigation is added to each content page which scrolls with the page to stay in view.
Re: QUnit test failures

I just looked into this, the tests themselves are bad, they try to use a regex to detect statements missing newlines. The test's regex incorrectly matches lines followed by a blank line:

```
> /\n [^\*].*:.*[^;{\/\*]\n/gm.test("\n    left: 250px;\n\n")
true
```