Represent the datetime objects as UTC in RFC3339 format

[fekad]

Fixing #288

According to the conversation in the issue, the response always returns with the UTC time in RFC3339 format even if it is not aligned to the actual timezone where the API is running.

[codecov]

Codecov Report

Merging #333 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   90.56%   90.57%   +0.01%     
==========================================
  Files          55       55              
  Lines        2268     2271       +3     
==========================================
+ Hits         2054     2057       +3     
  Misses        214      214              

Flag	Coverage Δ
#unittests	90.57% <100.00%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
optimade/server/routers/utils.py	97.72% <ø> (ø)
optimade/models/jsonapi.py	90.52% <100.00%> (+0.30%)	⬆️

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[CasperWA]

Thanks @fekad !

I've put it some comments/questions.

[CasperWA]

Thanks a bunch @fekad for the detailed analysis on top of this 👍