Move top-level pages into dirs

[Chris Warburton]

Currently, /projects.html comes from /projects.md and
/projects/index.html is a redirect to /projects.html. The same is
presumably true for blog pages as well.

We should switch this around, so /projects.md gets moved to
/projects/index.md and we generate /projects.html as a redirect to
/projects

This makes our URLs nicer, since we can avoid '.html' suffices.

It also makes site organisation nicer, since we don't have as many
pages sitting in the root directory.

Note that we need to prevent infinite recursion: when /projects/index.md
is rendered, it will be given the rendered contents of /projects. We
need to make sure that index.md is filtered out of that content!