Fix "add more metadata" fatal in Pholio

Summary: Ref T13332. This fix isn't terribly satisfying, but resolves the issue: this behavior may attempt to build HTML blocks with metadata after Javascript footer rendering has started. Use hsprintf() to flatten the markup earlier.

Test Plan: Put a T123 reference in the description of a Pholio image, then loaded a mock.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13332

Differential Revision: https://secure.phabricator.com/D20647