Add a sitemap-style footer

Fixes https://github.com/ocaml/ocaml.org/issues/257
Add build dependency on the footer for all pages
Add CompCert award

This ensures that the news https://github.com/ocaml/ocaml.org/issues/107
is not lost for the announcement feed to be created — see
https://github.com/ocaml/ocaml.org/issues/233
Center the footer columns

Fixes https://github.com/ocaml/ocaml.org/issues/257
Fix breadcrumb navigation when no index file is present
Adjust the line-height for the titles of the planet
Merge branch 'master' of https://github.com/ocaml/ocaml.org