Diffusion - move some DiffusionRequest queries to occur over Conduit

Summary: Ref T2784. This one was a wee bit complicated. Had to add PhabricatorUser and concept of initFromConduit (or not) to DiffusionRequest.

Test Plan: foreach repo, visited CALLSIGN and clicked a commit and verified they laoded correctly. Hacked code to hit NOT via Conduit and repeated tests to great success.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5928