Double escape %N (%%N) for pht in Releeph project editor

Summary: Oops, `pht()` interpolates percent escape sequences, but this is a literal "%N".

Test Plan: Edit a Releeph project; notice no errors.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5897