R1:48a3760814d6

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…