fix(api): register responses with apispec using components.response()

[jnahmias]

responses should be registered with apispec using components.response();
not by directly modifying the _reponses dict, which is internal to the
ApiSpec.Components class.

Also, handle duplicate response registrations gracefully.

[codecov]

Codecov Report

Merging #1881 (243041c) into master (93a446c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1881   +/-   ##
=======================================
  Coverage   77.64%   77.64%           
=======================================
  Files          70       70           
  Lines        8556     8559    +3     
=======================================
+ Hits         6643     6646    +3     
  Misses       1913     1913           

Flag	Coverage Δ
python	77.64% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flask_appbuilder/api/__init__.py	96.41% <100.00%> (+0.01%)	⬆️

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