Correct a bug where milestone "spacePHID" columns could become desynchronized

Summary:
Depends on D20041. See PHI1046. If you do this:

• Create a parent project called "Crab" in Space 1.
• Create a milestone called "Left Claw".
• Shift "Crab" to Space 2.
• Create a milestone called "Right Claw".

...you currently end up with "Left Claw" in the wrong spacePHID in the database. At the application level it's in the correct space, but when we WHERE ... AND spacePHID IN (...) we can incorrectly filter it out.

Test Plan:

• Did the above setup.
• Saved "Crab", saw the space fix itself.
• Put things back in the broken state.
• Ran the migration script, saw things fix themselves again.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: aeiser, PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D20063