fix(app): Use correct URLs for loading files that only exist outside of OpenNeuro

[nellh]

This resolves some cases where the file viewer would only load a file if OpenNeuro had a copy of it. Instead we always use the export URL unless one does not exist, falling back to OpenNeuro's file access API if needed before showing a 404 for a file that cannot be found in either case.

[codecov]

Codecov Report

Attention: Patch coverage is 13.33333% with 52 lines in your changes missing coverage. Please review.

Project coverage is 44.04%. Comparing base (2a391d3) to head (d803b54).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...uro-app/src/scripts/dataset/files/file-display.jsx	10.90%	49 Missing ⚠️
...nneuro-app/src/scripts/dataset/routes/snapshot.tsx	0.00%	2 Missing ⚠️
...pp/src/scripts/dataset/routes/tab-routes-draft.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3144      +/-   ##
==========================================
- Coverage   44.07%   44.04%   -0.04%     
==========================================
  Files         593      593              
  Lines       37760    37788      +28     
  Branches     1124     1124              
==========================================
+ Hits        16643    16644       +1     
- Misses      20915    20942      +27     
  Partials      202      202              

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