Split downloads.json into separate json files for each architecture

We currently have a downloads.json file containing the latest version of Tor Browser, and download links for all architectures.

In some cases however, some releases are only made for some architectures, while other architectures are staying on an older release. In that case the downloads.json file only contains the latest version, and some of the architectures are missing from the file.

To avoid that I think we can split downloads.json into separate json files for each architecture. We can then redirect each file to the right directory when needed.