R1:b27c2ed6d1e6

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…