User mapping - Policy Updates

For the improved user contribution mapping, contributions will be assigned to placeholder users, a special type of user with limited access that resembles the user in the source instance.

On #443533 (closed), we will create all the logic to create such users.

In this issue, we will ensure the user_type: placeholder can not perform any actions, especially sign-in and use the API.

Note: This issue doesn't need to be blocked by #443533 (closed). Define the user_type: placeholder like it will be done by the other issue and use the type to define/update the policies.