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 the column.

Test Plan: We run this on my company's internal fork and it seems to work okay. I don't think any other conduit methods anywhere have tests (???), but if you can point me at one I'm glad to write a unit test!

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: 20after4, speck, tobiaswiese

Differential Revision: https://we.phorge.it/D25018