contrib: Add systemd units for indexer

An example systemd service unit for the indexer and an accompanying
timer unit.

contrib/dspace-statistics-indexer.timer

@@ -0,0 +1,12 @@
+[Unit]
+Description=CGSpace Statistics Indexer
+
+[Timer]
+# twice a day, at 6AM and 6PM
+OnCalendar=*-*-* 06:00:00,18:00:00
+# Add a random delay of 0–3600 seconds
+RandomizedDelaySec=3600
+Persistent=true
+
+[Install]
+WantedBy=timers.target