"Relativise" pages during their build

[Chris Warburton]

At the moment we build each page separately, then we "relativise" them
(fix up their links) as part of building the site. This means we end up
relativising every page whenever the site changes, which is wasteful.

Instead, we should relativise either as part of the rendering process,
or else as a wrapper around each rendered page. The site is then a
collection of these relativised pages, so a change in the site doesn't
cause a rerun of all the relativisers.