Support an "authorPHIDs" constraint for "transaction.search"

Summary: Ref T13255. The "transaction.search" API method currently does not support author constraints, but this is a reasonable thing to support.

Test Plan: Queried transactions by author, hit the error cases.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13255

Differential Revision: https://secure.phabricator.com/D20208