Add a "default view" policy to Differential

Summary:
Ref T603. Allows the Differential view policy to be configured with a default.

I've omitted "edit" because I want to wait and see how comment/comment-action policies work out. I could imagine locking "edit" down to only the owner at some point, and providing a wider "interact" capability, or something like that, which would cover accept/reject/commandeer. Users in this group could still edit indirectly by commandeering first.

Test Plan: Created new revisions from the CLI and conduit.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7269