Fix server.js to support the single-page-app mode

[julienw]

No description provided.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.86%. Comparing base (45e9beb) to head (4132710).
Report is 1 commits behind head on beta.

Additional details and impacted files

@@           Coverage Diff           @@
##             beta     #651   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files          65       65           
  Lines        1404     1404           
  Branches      241      241           
=======================================
  Hits         1374     1374           
  Misses         28       28           
  Partials        2        2           

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