nixos/graphite-service: simplify options

This commit adds a 'services.graphite.enable' option that enables both
the carbon-cache backend and the webUI frontend. (Both sub-services can
still be disabled through their respective .enable options.)

The graphite service has so many options that I find it nice to be able
to get something working with just a .enable option.

Also extend some option documentation and add a couple of assertions.