Only show Service Desk email address to project members

What does this MR do?

Non-members shouldn't see the email directly, and they shouldn't see a message about 'your users' as they aren't their users.

Relates to https://gitlab.com/gitlab-org/gitlab-ee/issues/13554.

Screenshots

As member:

As non-member:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.