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