Summary: Depends on D18951. Ref T13049. When we export to CSV or plain text, add a header row in the first line of the file to explain what each column means. This often isn't obvious with PHIDs, etc. JSON has keys and is essentially self-labeling, so don't do anything special. Test Plan: Exported CSV and text, saw new headers. Exported JSON, no changes. Reviewers: amckinley Reviewed By: amckinley Maniphest Tasks: T13049 Differential Revision: https://secure.phabricator.com/D18952