Make mystery man the default Gravatar image

wagtail/wagtailadmin/templatetags/gravatar.py

@@ -28,7 +28,7 @@ class GravatarUrlNode(template.Node):
         except template.VariableDoesNotExist:
             return ''
 
-        default = "blank"
+        default = "mm"
         size = int(self.size) * 2  # requested at retina size by default and scaled down at point of use with css
 
         gravatar_url = "//www.gravatar.com/avatar/{hash}?{params}".format(