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.
`def downloadCount...` is aligned with a line of that no longer exists.

Also, I encourage people to leave out comments unless absolutely necessary. I think the comment `// downloadCount is...` could be safely left out.