[Django] Use explicit app label

Otherwise, all generated labels would be namespaced as django_.

bigbluebutton/django/apps.py

@@ -8,6 +8,7 @@ from . import settings, util
 class BigbluebuttonConfig(AppConfig):
     name = "bigbluebutton.django"
     verbose_name = _("BigBlueButton Django integration")
+    label = "bigbluebutton"
 
     def ready(self):
         """Extend the User model with attribute shortcuts."""