R1:86fe020a97b8

Add global "push" policy to Repositories

Summary: No editing or view yet, just adds the schema and a policy default. Part of D7391.

Test Plan: `bin/storage upgrade`

Reviewers: hach-que, btrahan

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7415