nixos/mailman: don't keep secrets in the Nix store

This replaces all Mailman secrets with ones that are generated the
first time the service is run.  This replaces the hyperkittyApiKey
option, which would lead to a secret in the world-readable store.
Even worse were the secrets hard-coded into mailman-web, which are not
just world-readable, but identical for all users!

services.mailman.hyperkittyApiKey has been removed, and so can no
longer be used to determine whether to enable Hyperkitty.  In its
place, there is a new option, services.mailman.hyperkitty.enable.  For
consistency, services.mailman.hyperkittyBaseUrl has been renamed to
services.mailman.hyperkitty.baseUrl.