Add quarter_hourly to cron configuration

It is purely optional as there was no job set to run quarter hourly.

web/mailman-web/uwsgi.ini

@@ -26,6 +26,7 @@ attach-daemon = ./manage.py qcluster
 
 # Setup hyperkitty's cron jobs.
 unique-cron = -1 -1 -1 -1 -1 ./manage.py runjobs minutely
+unique-cron = -15 -1 -1 -1 -1 ./manage.py runjobs quarter_hourly
 unique-cron = 0 -1 -1 -1 -1 ./manage.py runjobs hourly
 unique-cron = 0 0 -1 -1 -1 ./manage.py runjobs daily
 unique-cron = 0 0 1 -1 -1 ./manage.py runjobs monthly