Merge with ocaml.org styling and layout

This patchset contains the necessary modifications to have the `opam2web` tool generate pages which fit into the styling and layout of ocaml.org.

The coordination between the two sites has been designed such that the server for opam.ocaml.org will checkout ocaml.org regularly and build the template that it requires to match the design of ocaml.org (ocaml/ocaml.org#512). This generated template will source assets via scheme-relative URLs off of ocaml.org. For this to work without mixed content warnings, ocaml/ocaml.org#399 is required.

These changes will demand a number of updates to the opam.ocaml.org runtime scripts to accommodate this new dependency on the ocaml.org repository.

May the force be with you.