Expose publication models

[daviddavis]

ref #4678
https://pulp.plan.io/issues/4678

Please be sure you have read our documentation on creating PRs:
https://docs.pulpproject.org/en/3.0/nightly/contributing/pull-request-walkthrough.html

[codecov]

Codecov Report

Merging #90 into master will increase coverage by 0.6%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #90     +/-   ##
=========================================
+ Coverage   42.27%   42.87%   +0.6%     
=========================================
  Files          21       22      +1     
  Lines         712      723     +11     
=========================================
+ Hits          301      310      +9     
- Misses        411      413      +2

Impacted Files	Coverage Δ
pulpcore/plugin/serializers.py	100% <ø> (ø)	⬆️
pulpcore/plugin/viewsets.py	100% <ø> (ø)	⬆️
pulpcore/plugin/models/content.py	81.81% <0%> (ø)

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