Summary: Fixes T12505. `PhabricatorProjectsMembershipIndexEngineExtension->materializeProject()` was incorrectly bailing early for milestone objects, which prevented milestone members from being calculated correctly. This was causing problems where (for example) an Owners package owned by a milestone wasn't being satisfied when a member of the milestone approved a revision.
Test Plan: Invoked migration, observed that a user's milestones correctly showed up when searched for. Also observed that accepting a revision on behalf of a milestone now satisfies Owners rules.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T12505
Differential Revision: https://secure.phabricator.com/D18033
