Upload archiving

[bakape]

• [ ] If a thread has more than 10k posts (configurable) all images on posts before the 10k mark should count as archivable.
• [ ] If an image is used only by archivable posts, it can be deleted from the server (not the thumbnail) and marked as archived.
• [ ] When uploading an image, that already is archived, allow unarchiving it by reuploading the image.
  • [ ] Account upload insertion record in image archivability checks.
• [ ] Send archival results over websockets to thread feed subscribers.
• [ ] Never archive OP images.