small typo about the filter:

before it was printing a class "[] input"
instead of "input"

app/authentification/templatetags/utils_templating.py

@@ -7,5 +7,5 @@ register = template.Library()
 def addcss(value, arg):
     css_classes = value.field.widget.attrs.get('class', '').split(' ')
     if css_classes and arg not in css_classes:
-        css_classes = '%s %s' % (css_classes, arg)
+        css_classes = '%s %s' % (*css_classes, arg)
     return value.as_widget(attrs={'class': css_classes})