Allow Conduit to query project slugs.

Summary: Ref T4418. Allow Conduit to query projects by their slugs.

Test Plan: This functionality mostly already existed, it just wasn't exposed to the Conduit endpoint.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4418

Differential Revision: https://secure.phabricator.com/D9456