Don't mark ContextMixin as abstract

- Django abstract bases seems to apply only for Models.

- Django itself does not use abtract marking for its own generic view
  mixins.

plinth/modules/users/views.py

@@ -48,9 +48,6 @@ class PlinthContextMixin():
         context['title'] = getattr(self, 'title', '')
         return context
 
-    class Meta:
-        abstract = True
-
 
 class UserCreate(PlinthContextMixin, SuccessMessageMixin, CreateView):
     form_class = UserCreationForm