Feature: Add mapping update endpoint

[syphax-bouazzouni]

a refactor of #18, and add an endpoint PATCH 'mappings/:mapping' to update/edit an existent/created mapping metadata.

[codecov-commenter]

Codecov Report

Merging #103 (8770b02) into master (3728fab) will decrease coverage by 0.15%.
The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   71.80%   71.64%   -0.16%     
==========================================
  Files          52       52              
  Lines        2841     2850       +9     
==========================================
+ Hits         2040     2042       +2     
- Misses        801      808       +7     

Flag	Coverage Δ
unittests	71.64% <56.25%> (-0.16%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
controllers/mappings_controller.rb	76.61% <22.22%> (-5.36%)	⬇️
helpers/mappings_helper.rb	100.00% <100.00%> (ø)

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