Create a Matrix channel when a team got created

apps/registration/auth.py

@@ -9,6 +9,6 @@ class CustomAuthUser(DjangoAuthUser):
     def attributs(self):
         d = super().attributs()
         if self.user:
-            d["matrix_username"] = f"corres2math_{self.user.pk}"
+            d["matrix_username"] = self.user.registration.matrix_username
             d["display_name"] = str(self.user.registration)
         return d