Just require hostName and domain instead of an extra publicIPv4 option.  Leave
behind a similar option, `publicAddress`, just in case it needs to be
overridden at some point, but stop supplying a value for it anywhere.

Put its contents in it and then use them from there instead of importing them
all over the rest of the code.

The default case was actually catastrophically bad for any actual deployment
since it would just lock everyone out of all the systems.

More flexibility and less dots in config files.
Implements #65.

Discern make-issuer.nix from issuer.nix better.

The IP address is given as an argument

Also put library code in a library directory

Also some refactoring to make this easier in the future, steps towards
unifying production and testing definitions, and start of docs for the whole thing.

Define testing grid and production grid with a function they can share.