This diff adds conduit methods for searching for legalpad documents and signatures. This is very helpful for auditing who's actually signed a document. It also fixes the "contributorPHIDs" constraint in the existing search engine.
In order to expose legalpad signatures through Conduit, this adds a phid column to the legalpad_documentsignature table. It includes a migration (in the style of many previous phid-adding migrations) to actually populate the column.