Depend on ZKAPAuthorizer's overlays for anything ZKAPAuthorizer requires.

It knows better than we do about its own dependencies.  Let it control them.

It is better than what we have here and duplicating it is sad.  ZKAPAuthorizer
also has much tighter binding to Tahoe-LAFS version than anything in this
repository so it makes sense to let it dictate that detail.

Also bump the version being used.  This will be important in a minute.

Add NixOS and morph configuration for the testing and production grids.

Configurable public address

100TB NixOS configuration

Also open the firewall!  Yea.  That is a pretty important thing.

There will be no staging grid and the other two nodes in this grid went
elsewhere.

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

There is not going to be a staging grid.
There will be a testing grid and a production grid.
The testing grid will be loosey goosey.
The production grid will have incremental rollouts.

Also, comments.

This results in less repetition, less abstraction leakage, and provides more
power to the private-storage module (by exposing meaningful values instead of
Tahoe-LAFS config strings).

This is a bricked machine in the data center where running NixOS is hard.