Support export of revisions to Excel/CSV/JSON/etc

Summary: Ref T13210. See PHI806. This enables basic export of revisions into flat data formats. This isn't too fancy, but just covers the basics since the driving use case isn't especially concerned about getting all the fields and details.

Test Plan: Exported some revisions into JSON, got sensible output.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19743