Convert study attachment sources

Convert an uploaded original into the study file it becomes, and attach it: a still picture becomes a JPEG (a WebP when it has transparency) within 2,048 pixels and 3.75 MB, a moving picture or a video an H.264 MP4 a browser plays (copied as it stands when it already is one) of at most 3 minutes and 100 MiB, a sound an MP3 within 4 MiB, and slides or a document a PDF within 4 MiB. The study keeps only the converted file, named as the original with its new extension; the original is deleted. Returns the attachment, ready to reference from a study question; retrying with the same Idempotency-Key answers the same file without converting again. A video also returns its storyboard, which starts as preparing: read research_study_attachments_get_v2 until it is ready, refused or failed. A file Point11 cannot read, or one that cannot fit a study once converted, is refused with its reason.