REST plotting endpoint with regret plots

[Thomsch]

Description

This is the last foundational pull request for the REST API. It adds the /plots endpoint and serves regret plots as the first kind of plot available.

For more information, refer to #423.

Checklist

Tests

• I added corresponding tests for bug fixes and new features. If possible, the tests fail without the changes
• All new and existing tests are passing ($ tox -e py38; replace 38 by your Python version if necessary)

Documentation

• I have updated the relevant documentation related to my changes

Quality

• I have read the CONTRIBUTING doc
• My commits messages follow this format
• My code follows the style guidelines ($ tox -e lint)

[codecov-commenter]

Codecov Report

Merging #438 into develop will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #438      +/-   ##
===========================================
+ Coverage    48.95%   49.01%   +0.06%     
===========================================
  Files           80       81       +1     
  Lines        13640    13657      +17     
  Branches       329      329              
===========================================
+ Hits          6677     6694      +17     
  Misses        6946     6946              
  Partials        17       17              

Impacted Files	Coverage Δ
tests/functional/serving/test_plots_resource.py	100.00% <100.00%> (ø)

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[bouthilx]

👌 👌 🙂