AODAAC challenge captcha

Solving #1214 and #1216 by adding a challenge (captcha) to AODAAC downloads.

This work also unifies AODAAC and Gogoduck controllers into AsyncDownloadController which will handle both and route to either service depending on the `aggregatorService` passed to it. I think now the test coverage is also better.
This variable is no longer used (really) so it can be removed