R1:493196e16a6f

Add Conduit method for getting Diffusion lint messages

Test Plan: /conduit/method/diffusion.getlintmessages/, ran it, saw results.

Reviewers: epriestley, wez

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2038

Differential Revision: https://secure.phabricator.com/D3932