users: fix crash when creating users with non ASCII characters

This was already changed by the cleaned up on the default branch in
330c671dd451 but happened to also fix this issue on the stable branch.

kallithea/model/user.py

@@ -191,7 +191,7 b' class UserModel(BaseModel):'
         # notification to admins
         subject = _('New user registration')
         body = (
-            'New user registration\n'
+            u'New user registration\n'
             '---------------------\n'
             '- Username: {user.username}\n'
             '- Full Name: {user.full_name}\n'