When rendering the "you were invited" header, query the inviting user with the omnipotent viewer

Summary: Fixes T11982. If an install is not public, the registering user may not be able to see the inviting user.

Test Plan: {F2097656}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11982

Differential Revision: https://secure.phabricator.com/D17015