WIP: Blank page during deployment - secure a bucket with assets

Created by: oleggromov

This is to track a step of https://github.com/sourcegraph/sourcegraph/issues/30536.

Plan

1. Create a bucket for UI assets, which should be publicly available
2. Make it available under assets.sourcegraph.com
3. Connect to Cloudflare
4. Make sure old asset versions are removed after 6 months