Index Project milestones to accurately reflect milestone membership

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