Add cron job to remove old django sessions (#17)

cron/README.md

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+
+# How to 
+
+Run `sh add_cronjob.sh` to add the `remove_old_django_session.sh` to crontab.
+
+If you get a message 'no crontab for [user]' make sure you have a crontab by running this command : 
+```
+crontab -e
+```

cron/add_cronjob.sh

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+#!/bin/bash
+
+crontab -l > remove_django_session_cronjob
+
+echo "0 0 * * * sh ${PWD}/remove_old_django_session.sh" >> remove_django_session_cronjob
+
+crontab remove_django_session_cronjob
+
+rm remove_django_session_cronjob

cron/remove_old_django_session.sh

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+#!/bin/bash
+
+DOCKER_DB=$(docker ps --format '{{.Names}}' | grep qgis-feed_postgis | head -1)
+DB_USER=postgres
+DB_NAME=qgisfeed
+INTERVAL=1
+
+docker exec $DOCKER_DB su - $DB_USER -c "psql -d $DB_NAME -c \"DELETE FROM 
+    DJANGO_SESSION WHERE expire_date < now() - interval '$INTERVAL days';\""