Expose "isDraft" and "holdAsDraft" revision properties over Conduit

Summary:
Ref T13195. See PHI861. The "+ Draft" flag is not currently exposed over the API, but seems stable enough to expose.

Also expose the "hold as draft" flag, normally arc diff --draft.

Today, you can get "+ Draft" with some other state by:

• abandoning a draft revision ("Abandoned + Draft"); or
• using arc diff --plan-changes with an older arc version ("Changes Planned + Draft").

Test Plan: Called differential.revision.search via Conduit and got sensible-looking results for revisions in various states.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19648