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/accounts/management/commands/year_revalidation_users.py

@@ -211,8 +211,7 @@ class Command(BaseCommand):
                                     if supervisiontrack.expiration_date.date() - now.date() == datetime.timedelta(days=expiration_reminder):
                                         warned_count += 1
 
-                                        temp_url = TemporaryUrl()
-                                        temp_url._generate_temporary_url(TemporaryUrl.YEARREVALIDATION, supervisiontrack)
+                                        temp_url = TemporaryUrl.objects.create_temporary_url(TemporaryUrl.YEARREVALIDATION, supervisiontrack)
 
                                         context = {
                                             'user': user,