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.
Can you split it up for readability
```javascript
oae.api.util.template().render($navTemplate, {
    'navs': navEntries,
    'navTitle': navTitle
}, $navigationWrapper);
```