Refactor Member#add_user for GitLab EE

What does this MR do?

GitLab EE extends Member#add_user by adding some LDAP specific flags. Prior to these changes, the only way this could be done was by modifying Member#add_user in place. This could lead to merge conflicts whenever a developer wants to change this method.

To resolve this issue, the logic that EE extends has been moved into a separate method with the appropriate arguments. This allows EE to extend the logic by prepending the method using an EE specific module.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8471

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8244

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides