Import link study attachments

Attach a study file by its link instead of uploading it: Point11 checks the link is safe, fetches the file it points to, and admits it exactly as an upload (an image or a PDF up to 4 MiB, MP3 or WAV audio up to 4 MiB, a plain text, Markdown or CSV file up to 256 KB, or an MP4 or WebM video up to 100 MiB whose server states its size). The type is the one the link's server names, else the one the file's name names, and the bytes must prove it. A web page is refused: link the file itself. Returns the attachment, ready to reference from a study question; retrying with the same Idempotency-Key answers the same file without fetching the link again. When the link's safety check cannot run, it answers 503: try again. A video also returns its storyboard, which starts as preparing: read research_study_attachments_get_v2 until it is ready, refused or failed.