Give designate-milestone +2/+A on stable/*

This was removed in Ie14946e5f749a0c91e382dac0b06abefd4234ee2, where
we incorrectly assumed the exclusiveGroupPermissions line from
All-Project would not be inherited, and that the refs/heads/*
permissions would be inherited.

Change-Id: Ibfa5b9d6519aa7e22d1ec1196d361c633f4d321a
Merge "Give designate-milestone +2/+A on stable/*"