[abp-backend] Move updateDownloadLinks script to the update server

[trev]

Background

Currently sitescripts.extensions.bin.updateDownloadLinks script is running on the main server and dumping generated update manifest into its root directory.

What to change

We have update.adblockplus.org that should host the update files, it makes sense to run sitescripts.extensions.bin.updateDownloadLinks there. The script currently runs twice per hour, on the update server it can run more frequently. Note that the script itself also needs to be adjusted - checking whether a file exists in the downloads repository is no longer sufficient, it also needs to verify that this file made it to the downloads server.