R1:e03103f34920

Return milestone information in project.search

Summary:
Ref T12074.

- `project.search` now returns milestones by default.
- A new constraint, `isMilestone`, allows filtering to milestones, non-milestones, or both (API and web UI).
- `project.search` now returns a milestone number for milestones, or `null` for non-milestones.

NOTE: Existing custom saved queries in projects which previously did not return milestones now will. I expect this to have little-to-no impact on users, and these queries are…