Bugfix for superevent date ID regex

Changing \d+ to \d{6} in superevent date ID regex.  This prevents
ValueErrors from occurring when people try to go to URLs like
/superevents/view/S0001.

gracedb/superevents/api/view_templates.py

@@ -7,7 +7,7 @@ from .views import SupereventViewSet, SupereventEventViewSet, \
 
 # Placeholder parameters for getting URLs with reverse
 PH = {
-    SupereventViewSet.lookup_field: 'S1234', # superevent_id
+    SupereventViewSet.lookup_field: 'S800106', # superevent_id
     SupereventEventViewSet.lookup_field: 'G1234', # graceid
     SupereventLabelViewSet.lookup_field: 'LABEL_NAME', # label name
     SupereventLogViewSet.lookup_field: '3333', # log number (N)

gracedb/superevents/models.py

@@ -42,7 +42,7 @@ class Superevent(CleanSaveModel, ModelToDictMixin, AutoIncrementModel):
     """
     DEFAULT_ID_PREFIX = 'S'
     GW_ID_PREFIX = 'GW'
-    ID_REGEX = r'(({0})(\d+)([a-z]*)|({1})(\d+)([A-Z]*))'.format(
+    ID_REGEX = r'(({0})(\d{{6}})([a-z]*)|({1})(\d{{6}})([A-Z]*))'.format(
         DEFAULT_ID_PREFIX, GW_ID_PREFIX)
     DATE_STR_FMT = '%y%m%d'
     AUTO_FIELD = 'base_date_number'