Accessibility: aria-label added to navigation links

Current behavior: Screenreader just reads ‘link’ when landing on the
previous and next navigation links, giving the user no indication of
what the link does or where it goes. New Behaviour: Screenreader reads
“Previous page: {page title}” and “Next page: {page title}”
respectively.
Merge pull request #213 from ianmcburnie/accessibility

Accessibility: aria-label added to navigation links