Make creation of downloads.json optional

In tools/update-responses/update_responses we generate a downloads.json file containing the list of bundles available for download. However we don't need to create that file for nightly builds, and only have the .mar files available when running this script for the nightly updates. So we should add an option to make creation of downloads.json optional.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information