R1:16537f7b322f

Support filtering feed transactions by object type

Summary: Depends on D20533. Allow querying for transactions of a specific object type, so you can run queries like "Show all edits to Herald rules between date X and Y".

Test Plan: {F6463478}

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20534