Implement common infrastructure fields as export extensions

Summary:
Depends on D18959. Ref T13049. Provide tags, subscribers, spaces, and created/modified as automatic extensions for all objects which support them.

(Also, for JSON export, be a little more consistent about exporting null instead of empty string when there's no value in a text field.)

Test Plan: Exported users and tasks, saw relevant fields in the export.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18960