Improve automatic emails with temporary urls

This MR targets the improvement for automatic emails concerning validation of new users by supervisors/re-validation of users.

Fixes #542 (closed)

Fixes #550 (closed)

beat/web/ui/registration/models.py

@@ -144,8 +144,7 @@ class RegistrationManager(models.Manager):
                         parsed_url.hostname,
                     )
 
-                    temp_url = TemporaryUrl()
-                    temp_url._generate_temporary_url(TemporaryUrl.VALIDATION, supervisiontrack)
+                    temp_url = TemporaryUrl.objects.create_temporary_url(TemporaryUrl.VALIDATION, supervisiontrack)
 
                     context = {
                         "supervisor": supervisor_user,