Enrich "draft", "summary", and "testPlan" transactions from Differential in "transaction.search"

Summary:
See PHI1232, which describes a reasonable use case for wanting information about the "draft" ("Hold as Draft / Do Not Auto-Promote") flag.

Also, flesh out "testPlan" and "summary". It's possible these "blob of remarkup" fields might have metadata some day (e.g., a rendered version or a list of PHIDs or something), but we could add more keys, and we already have some other transactions which work like this.

Test Plan: Used "transaction.search" to fetch these transaction types, saw type information and metadata.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20507