Build a rough transaction-level view of Feed

Summary:
Ref T13294. An install is interested in a way to easily answer audit-focused questions like "what edits were made to any Herald rule in Q1 2019?".

We can answer this kind of question with a more granular version of feed that focuses on being exhaustive rather than being human-readable.

This starts a rough version of it and deals with the two major tricky pieces: transactions are in a lot of different tables; and paging across them is not trivial.

To solve "lots of tables", we just query every table. There's a little bit of sleight-of-hand to get this working, but nothing too awful.

To solve "paging is hard", we order by "<dateCreated, phid>". The "phid" part of this order doesn't have much meaning, but it lets us put every transaction in a single, stable, global order and identify a place in that ordering given only one transaction PHID.

Test Plan: {F6463076}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13294

Differential Revision: https://secure.phabricator.com/D20531