R1:a4948ec800e9

Add conduit endpoints for querying legalpad

Summary:
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…