R1:135280be9e1f

Support HTML5 / Javascript chunked file uploads

Summary:
Ref T7149. This adds chunking support to drag-and-drop uploads. It never activates right now unless you hack things up, since the chunk engine is still hard-coded as disabled.

The overall approach is the same as `arc upload` in D12061, with some slight changes to the API return values to avoid a few extra HTTP calls.

Test Plan:
- Enabled chunk engine.
- Uploaded some READMEs in a bunch of tiny 32 byte chunks.
- Worked out of the box in…