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.

conf/systemd/webvirt-backup.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Daily WebVirtCloud panel backup
+
+[Timer]
+OnCalendar=*-*-* 03:15
+# a backup missed while the server was off runs at the next boot
+Persistent=true
+RandomizedDelaySec=10min
+
+[Install]
+WantedBy=timers.target