added filter for displaying visit number from zero or one depending on settings

smash/web/templatetags/filters.py

 # See: http://stackoverflow.com/a/18962481
 from django import template
+from django.conf import settings
 from django.forms import CheckboxSelectMultiple, CheckboxInput
 from django.utils.safestring import mark_safe
 import datetime
@@ -46,3 +47,11 @@ def render_appointments(statistics, appointment_type):
 def timestamp(value):
     epoch = datetime.datetime.utcfromtimestamp(0)
     return (value.replace(tzinfo=None) - epoch).total_seconds()
+
+
+@register.filter(name='display_visit_number')
+def display_visit_number(visit_number):
+    if getattr(settings, "SHOW_VISITS_FROM_ZERO", False):
+        return (visit_number - 1)
+    else:
+        return visit_number
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