fix(server): Don't cache the default dataset_description response

[nellh]

This prevents a bug where the inaccessible dataset_description.json response could be cached by moving it after cache resolution. This seems to be the main cause of #2707

A minor change here is to drop superagent for this request (uses native fetch instead).

[codecov]

Codecov Report

Merging #2753 (724f240) into master (c6f70fb) will decrease coverage by 0.01%.
The diff coverage is 52.17%.

@@            Coverage Diff             @@
##           master    #2753      +/-   ##
==========================================
- Coverage   76.04%   76.03%   -0.01%     
==========================================
  Files         411      411              
  Lines       66990    66993       +3     
  Branches     2039     2038       -1     
==========================================
- Hits        50942    50938       -4     
- Misses      15997    16004       +7     
  Partials       51       51              

Impacted Files	Coverage Δ
...ckages/openneuro-server/src/datalad/description.js	74.32% <52.17%> (-4.30%)	⬇️

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