Add default View, Edit, and Join Policies to Projects

Summary: Fixes T6567, lets admins set a default policy for new Projects.

Test Plan: Changed the default Policy, created a new Project. Project had correct defaults.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6567

Differential Revision: https://secure.phabricator.com/D10890