R1:be2b8f4bcb62

Support querying projects by "Root Projects" in the UI, and "min/max depth" in the API

Summary:
Fixes T13441. Internally, projects can be queried by depth, but this is not exposed in the UI.

Add a "Is root project?" contraint in the UI, and "minDepth" / "maxDepth" constraints to the API.

Test Plan:
- Used the UI to query root projects, got only root projects back.
- Used "project.search" in the API to query combinations of root projects and projects at particular depths, got matching results.