Restore export API
Restore export API

Commit 477d71e0b90d1 was based on an older version of export.js, which
means it reverted a few changes that were made previously.

Fixes #2328

wekan/wekan,GhassenRjab/wekan,wekan/wekan,wekan/wekan,wekan/wekan,wekan/wekan,GhassenRjab/wekan,GhassenRjab/wekan