settings: Disable DisallowedHost error emails by default.

These error messages are pretty spammy because most servers on the
public Internet receive some amount of HTTP(S) scanning traffic.

We still log them, just don't email the administrators.

zproject/settings.py

@@ -937,6 +937,10 @@ LOGGING = {
             'level':    'WARNING',
             'propagate': False,
         },
+        'django.security.DisallowedHost': {
+            'handlers': ['file'],
+            'propagate': False,
+        },
         ## Uncomment the following to get all database queries logged to the console
         # 'django.db': {
         #     'handlers': ['console'],