Ref T12074.

• Adds a new "parent" property on main results. This shows an abbreviated version of the project's parent, or null if the project is a root project.
• Adds a new "ancestor" attachment to pull the entire ancestor list.
• Adds a new "depth" property on main results.
• You can use "parent" or "depth" to tell if a project is a subproject or not.

These attempt to balance convenience, power, and performance: the full ancestor list can be big so I made it an attachment, but the other stuff isn't too big and is cheap and seems reasonable to always include.

In API results:

• Saw null parent (root projects) and non-null parent (subprojects/milestones).
• Used "ancestors" attchment, got full list of ancestors.
• Saw appropriate "depth" values.