• Add a DELETE API to the media store (the uploader is recorded, and it's auth'd, so this seems feasible)
• Allow media uploads to say "don't cache this" on media
• Allow the invitee to send a receipt to the inviter when they have finished with the key bundle
• Inviter deletes the media when we receive the receipt

Alternatively: MSC4425

Part of element-hq/element-meta#2829