feat(helm): add nginx ConfigMap for reana-ui security headers

[CameronMcClymont]

Adds an nginx-config.yaml ConfigMap to make reana-ui security headers customisable via helm upgrade.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.86%. Comparing base (4f2ba90) to head (2c0048c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #945   +/-   ##
=======================================
  Coverage   29.86%   29.86%           
=======================================
  Files          26       26           
  Lines        2823     2823           
=======================================
  Hits          843      843           
  Misses       1980     1980           

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