R1:32d8d675357c

Support resuming JS uploads of chunked files

Summary: Ref T7149. We can't compute hashes of large files efficiently, but we can resume uploads by the same author, with the same name and file size, which are only partially completed. This seems like a reasonable heuristic that is unlikely to ever misfire, even if it's a little magical.

Test Plan:
- Forced chunking on.
- Started uploading a chunked file.
- Closed the browser window.
- Dropped it into a new window.
- Upload resumed //(!!!)//…