Enrich "priority" transactions in Maniphest for "transaction.search"
Summary:
Ref T13187. See <https://discourse.phabricator-community.org/t/task-priority-change-info-missing-in-firehose-webhook/1832/2>. We can reasonably enrich these transactions.
Since priorities don't have unique authorative string identifiers, I've mostly mimicked the `maniphest.search` structure.
Test Plan: Called `transaction.search` on tasks which were: created normally, created with a priority change, saw a priority change after creation. All the output looked useful and sensible.
Reviewers: amckinley
Maniphest Tasks: T13187
Differential Revision: https://secure.phabricator.com/D19599