Add custom serializers for "_artifact" vs "_artifacts"

[dralley]

Remove boilerplate from the plugins by defining new, more specialized
serializers for the single-artifact-per-content case, the
multiple-artifacts-per-content case, and the no-artifacts-per-content
case.

Required PR: pulp/pulp_file#159
Required PR: pulp/pulp#3846
Required PR: pulp/pulp-smash#1165

re: #4340
https://pulp.plan.io/issues/4340
re: #4366
https://pulp.plan.io/issues/4366

Pull Requests

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[codecov]

Codecov Report

Merging #38 into master will decrease coverage by 0.72%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   40.55%   39.82%   -0.73%     
==========================================
  Files          22       20       -2     
  Lines         688      678      -10     
==========================================
- Hits          279      270       -9     
+ Misses        409      408       -1

Impacted Files	Coverage Δ
pulpcore/plugin/serializers.py	100% <ø> (ø)	⬆️
pulpcore/plugin/stages/association_stages.py	20.68% <0%> (ø)	⬆️
pulpcore/plugin/models/__init__.py
pulpcore/plugin/models/content.py

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[dralley]

This double-indentation was causing Travis to fail