Build using GNU make

This is a reasonably large change.

The monolithic jenkins.sh script now calls make, which selectively
rebuilds the nginx config and assets, only if the source files or
tools scripts upon which they depend have been touched.

The Makefile depends upon a per-site makefile generated from a
per-site yaml file, which are currently generated from data/sites.csv.

Mustache is used to generate the makefiles, and will be used from
now to template other configuration files.