feat(appsettings): the panel backs itself up every day

The only backups of the panel were the ones taken by hand before an
update. Now `manage.py backup` writes a new directory under backups/
with a consistent copy of the sqlite database (the backup API, while
the panel runs, checked with PRAGMA integrity_check), the secret key,
the worker's node_id, settings.py, the SSH keys of the panel's user
and, outside Docker, the commit. It keeps the newest 7 (--keep N) and
deletes older ones only after a good backup; a failed one leaves
nothing, and a second run while one runs fails. Files are mode 600,
directories 700: the database holds compute passwords and keys. With
PostgreSQL the database is left out.

It runs daily at 03:15: in Docker from the image's cron into a new
volume, webvirtcloud-backups; otherwise from webvirt-backup.timer,
which the installer and the manual steps install. A backup has the
layout of the update backups, so the restore steps take either.

docker-compose.yml

@@ -31,9 +31,13 @@ services:
       - webvirtcloud-data:/srv/webvirtcloud/data
       # Persistent SSH keys for compute node authentication
       - webvirtcloud-ssh:/var/www/.ssh
+      # Daily backups of both (manage.py backup)
+      - webvirtcloud-backups:/srv/webvirtcloud/backups
 
 volumes:
   webvirtcloud-data:
     driver: local
   webvirtcloud-ssh:
     driver: local
+  webvirtcloud-backups:
+    driver: local