Draft: Misskeyish Drive

Will resolve #2738

• internal:
  • Ability to list uploaded files by one user (Image and Document)
  • Ability to delete uploaded files (dedupe-aware)
  • Track attachment relationship between Objects
  • Atomicity when performing actions on files (:global)
• API
  • list media
  • provide information on where media is used
  • delete media
• [later] User quota for uploading, dynamically assigned using a couple of factors like post numbers, registration date, user tags?
• [later] Maybe: access control
• Migration code to scan all uploaded attachments and assign the corresponding upload path (would this be possible?)
  • Add ap ids to all upload objects
  • Create an UploadedFile for each one
  • Legacy ap-id-less upload objects: how to link them?