When predicting object policies for project milestones, adjust objects so they behave like milestones

Summary:
Ref T13462. Currently, when testing milestone edit policies during creation, the project object does not behave like a milestone:

• it doesn't have a milestone number yet, so it doesn't try to access the parent project; and
• the parent project isn't attached yet.

Instead: attach the parent project sooner (which "should" be harmless, although it's possible this has weird side effects); and give the adjusted policy object a dummy milestone number if it doesn't have one yet. This forces it to act like a milestone when emitting policies.

Test Plan:

• Set "Projects" application default edit policy to "No One".
• Created a milestone I had permission to create.
  • Before: failed with a policy error, because the project behaved like a non-milestone and returned "No One" as the effective edit policy.
  • After: worked properly, correctly evaluting the parent project edit policy as the effective edit policy.
• Tried to create a milestone I did not have permission to create (no edit permission on parent project).
  • Got an appropriate edit policy error.

Maniphest Tasks: T13462

Differential Revision: https://secure.phabricator.com/D20919