Support postponed lint status in creatediff conduit call

Test Plan:
Created diff with a postponed linter and the lint status set to such
via arcanist. Verified lint status showed as postponed in diff view.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1332

Differential Revision: https://secure.phabricator.com/D2932