opam.ocaml.org integration

This should prepare ocaml.org to generate a template for opam.ocaml.org and host the necessary assets. No user-visible changes should occur after application.

Please review.
Yes, they are, but opam2web uses cow uses xmlm which assumes it's talking to an XML parser which would treat the content of `<script>` as PCDATA rather than CDATA which means that character entities are not parsed and all kinds of nasty junk can be included verbatim in the body of a `<script>`. The HTML `<script>` parsing rules are gnarly and the New Web Order (WHATWG) has decided in their glue-sniffing wisdom to **not** provide a way to treat the content of `<script>` like the content of any other tag if you want to ship `<script>` with character entities embedded...

This is all a long way to say that xmlm outputs `&quot;` in PCDATA (everything) so that it doesn't have to distinguish the quoting context (it uses `"` for attribute quoting and so does not want to inject new `"` characters).

If we just avoid `<`, `>`, `&`, and `"` in embedded `<script>` and `<style>` for now, we'll be ok...