This commit integrates WriteFreely as a systemd service, set up as a
single-user instance by default (as is probably appropriate for a
home-server setup); a default administrator is set up, and whoever
is managing the home-server is expected to update the username and
password after first login.
Though WriteFreely expects to have a hostname set up for the instance,
we do not listen on any specific hostname by default. It is expected,
rather, that the `nginx-proxy-http` service is used with a drop-in for
using the correct `writefreely` upstream.
Configuration for this will continue to evolve as required.
This is a basic implementation on top of the venerable `rss2email`
script, and is intended to be driven by a timer and the
`rss2email-subscribe` service, which manages the subscribed feeds.
