R1:b2cfcda114a4

Provide detailed information about reviewer changes in "transaction.search"

Summary:
See PHI1722, which requests transaction details about reviewer changes.

This adds them; they're structured to be similar to "projects" and "subscribers" transactions and the "reviewers" attachment on revisions.

Test Plan: {F7410675}

Differential Revision: https://secure.phabricator.com/D21207