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

src/applications/differential/controller/DifferentialRevisionEditController.php

@@ -79,6 +79,13 @@ final class DifferentialRevisionEditController
       if ($repository_field) {
         $repository_field->setValue($request->getStr($repo_key));
       }
+      $view_policy_key = id(new DifferentialViewPolicyField())->getFieldKey();
+      $view_policy_field = idx(
+        $field_list->getFields(),
+        $view_policy_key);
+      if ($view_policy_field) {
+        $view_policy_field->setValue($diff->getViewPolicy());
+      }
     }
 
     $validation_exception = null;