Meets #14377: Align copy project with DTAG privacy guidelines

[``* `#14377` Align copy project with DTAG privacy guidelines``](https://www.openproject.org/work_packages/14377)
What about a ``find_by_user_id``? So this would become:

 ```ruby
member = project.memberships.find_by_user_id(new_member.user_id)
```