Abstracting and expanding keyboard navigation functionality

Per #119, moving from binding keyboard shortcuts to links embedded in
HTML to binding them to link rel navigation tags. This will allow
keyboard navigation to work anywhere in the site (i.e., structural
units) where link rel navigation tags are present. Also, added support
for "up" (per #151), and moved to requiring the use of the control key
as a modifier, to avoid preventing people from scrolling within their
browser.