Support export of feed transactions to CSV/Excel/etc

Summary: Depends on D20534. Ref T13294. Add export support so you can dump these out, print them on paper, notarize them, and store them in a box under a tree or whatever.

Test Plan: Exported transactions to a flat file, read the file.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13294

Differential Revision: https://secure.phabricator.com/D20535