fix(server): Set content-length and use faster fetch API for file transfers

[nellh]

This replaces the superagent variant of getFile with the fetch equivalent. Testing locally with a 10GB nifti this is a little faster than the superagent version, which had been used for performance reasons originally.

# Superagent
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9.8G    0  9.8G    0     0   975M      0 --:--:--  0:00:10 --:--:--  959M
# Fetch
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  9.8G    0  9.8G    0     0  1392M      0 --:--:--  0:00:07 --:--:-- 1412M

This change also adds a content-length header to the response, allowing an HTTP client to detect errors when the connection is closed early.

This may fix #2798, I think this is worth trying in staging (it isn't reproducible locally).

[effigies]

No idea if this is sensible or plausible, but would it work to redirect to the S3 URL for snapshots? (Will look at this PR tomorrow.)

[nellh]

No idea if this is sensible or plausible, but would it work to redirect to the S3 URL for snapshots? (Will look at this PR tomorrow.)

I think I have a quick improvement for that in mind, I'll make that separately.

[effigies]

This reads sensibly. If I understand it correctly, it takes an external URL, translates it to an internal URL and then proxies the internal stream back out to the external request.

Probably want to remove L11-L12 comment.