Improve the infrastructure scripts

[Chris00]

• The infrastructure scripts should be under version control.
• The scripts should be improved to open an issue when something goes wrong. (Github will then forward mails.)
• A daily rebuild from scratch with various compilers should take place (with Github error reporting). The idea is to perform that the site is easily buildable by users with the recommended versions of the compiler.
• Automatically report (opening a Github issue) when there is a problem with the package list.

[Chris00]

See also ocaml/infrastructure#4