Add a Phrequent curtain extension

Summary: Fixes T10546. Some day, decades from now, we can revisit this when we iterate on Phrequent. Just don't regress for no real reason in the meantime, since it's easy enough to keep it working in reasonable shape.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10546

Differential Revision: https://secure.phabricator.com/D15461