Differential - allow setting viewPolicy from web ui during diff creation process

Summary: Fixes T6152, T6237. This introduces a viewPolicy column to the DifferentialDiff, and re-jiggers the DifferentialDiff policy implementation such that things behave as before once associated with a revision, else use the DifferentialDiff policy column value.

Test Plan: made a diff with a non-standard view policy and noted that policy was still selected in the revision step. arc lint.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6237, T6152

Differential Revision: https://secure.phabricator.com/D10875