problem

In TRANSLATE-3117 a download progress was specified - this is missing in implementation.

There are basically two possibilities:

1. implementation like in LangeresourceinstanceController::tbxexportAction
   • problem: no direct access to the waiting loop in blocking worker code
   • workaround: a callback to the progress calculation of the workers?
2. since export is running in a worker:
   • implementation with a progress bar in UI
   • define download URL / tmp file on main request
   • long polling on the worker process (to react on errors)
   • if ready download from above defined URL

Please also fix the tests, since they are not running anymore after my changes.