Allow searching for users by email with the API, for owners of an org/project

Please see https://github.com/gitlabhq/terraform-provider-gitlab/issues/839 for a summary of the issue. Essentially there was a change to the API yesterday whereby users cannot be searched for by email address.

I understand the privacy concerns, and I have asked our colleagues to set their email to public as a workaround for now.

However I believe it makes sense to allow an org/project owner to search for users, within that org/project, using the email address, for the reasons outlined in my Terraform provider issue. Only Admins can search using the email attribute, but Admins are exclusively Admins of Gitlab SaaS, rather than owners of orgs/projects hosted on Gitlab SaaS.

In our case where users are SCIM provisioned and login in through SSO, it makes little sense for this attribute to be hidden to an owner, as the user is tied to SSO, which is tied to their email address.