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 easy to correct, but I'll note this in changelogs.

Test Plan:
  - Ran various queries with `project.search` and in the web UI, searching for milestones, non-milestones, and both.
  - Web UI default behavior (no milestones) is unchanged, but you can now get milestones if you want them.
  - Queried a milestone by ID/PHID via API.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12074

Differential Revision: https://secure.phabricator.com/D17153