Support basic export of user accounts

Summary:
Depends on D18934. Ref T13046. Add support for the new export flow to a second application.

My goal here is mostly just to make sure that this is general enough to work in more than one place, and exporting user accounts seems plausible as a useful feature, although we do see occasional requests for this feature exactly (like https://discourse.phabricator-community.org/t/users-export-to-csv/968).

The exported data may not truly be useful for much (no disabled/admin/verified/MFA flags, no external account data, no email addresses for policy reasons) but we can expand it as use cases arise.

Test Plan: Exported user accounts in several formats.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13046

Differential Revision: https://secure.phabricator.com/D18935