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