Sanitize the sites' download/update

When updating or downloading a site, even being the media optional or not, don't download media automatically. Notify when a download/update contains media, and show user all media for approving.

The issue names no files, tests, or entry points. Start by tracing the site's download and update flow, then identify how media is handled; done means media is not downloaded automatically, users are notified when media is present, and all media is shown for approval.

索引モデルが issue の本文から書いたものです。