Add systemd timer to generate daily history

Author: sakhnik

systemd/gallery-daily.service

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+[Unit]
+Description=Prepare "this day in past years" gallery
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/python /home/sakhnik/work/shotwell-view/daily.py
+User=sakhnik
+Group=users

systemd/gallery-daily.timer

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+[Unit]
+Description=Generate "this day in past years" gallery daily
+
+[Timer]
+OnCalendar=*-*-* 4:00:00
+Persistent=true
+
+[Install]
+WantedBy=timers.target