R1:0ec83132a8ec

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…