Decidim: Private exports can be downloaded through reusable links

Decidim is a participatory democracy framework. Prior to 0.30.9, from 0.31.0 before 0.31.5, and in 0.32.0.rc1 before 0.32.0.rc2, the normal download_your_data flow requires the requester to be logged in as the export owner, but the resulting Active Storage blob redirect URL can be replayed without authentication by anyone who obtains it. This is because Decidim::DownloadYourDataController#download_file authenticates the export owner but redirects to a signed Active Storage blob URL that is no longer bound to the owner session. This issue is fixed in versions 0.30.9, 0.31.5, and 0.32.0.rc2.