R1:e96cd29efff7

Reduce badness in viewing large files in the Diffusion web UI

Summary:
Fixes T8597. Second issue there is that if you look at a huge file in Diffusion (like `/path/to/300MB.pdf`) we pull the whole thing over Conduit upfront, then try to shove it into file storage.

Instead, pull no more than the chunk limit (normally 4MB) and don't spend more than 10s pulling data.

If we get 4MB of data and/or time out, just fail with a message in the vein of "this is a really big file".

Eventually, we could improve…