Redact Service Desk email address when viewed by non-authorized user

Proposal

• Service Desk issues are private by default
• Service Desk users can choose to make them public
• Doing so exposes the customer's email address to anyone who can view the project (everyone, if project is public)

In #347131 (closed) documentation was added to warn the Service Desk user that this will happen (see https://docs.gitlab.com/ee/user/project/service_desk.html#privacy-considerations).

This email address should be redacted for non-authorized users, allowing the documentation to be removed.

Note: Email address is exposed in both the header and in subsequent notes related to the issue.

Related to #347131 (closed).