Refactor FullUrlMixin get_domain to handle django.contrib.sites not in INSTALLED_APPS

[protoroto]

Description

Refactor FullUrlMixin get_domain to handle django.contrib.sites not in INSTALLED_APPS

References

Fix #192

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included (see docs)
• Usage documentation added in case of new features
• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a4de31) 97.81% compared to head (6eb7b64) 98.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #194      +/-   ##
===========================================
+ Coverage    97.81%   98.00%   +0.19%     
===========================================
  Files            8        8              
  Lines          548      552       +4     
  Branches        99       98       -1     
===========================================
+ Hits           536      541       +5     
  Misses           6        6              
+ Partials         6        5       -1     

Flag	Coverage Δ
unittests	97.82% <100.00%> (+0.19%)	⬆️

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[coveralls]

coverage: 98.913% (+0.008%) from 98.905%
when pulling 6eb7b64 on bugfix/issue-192-refactor-fullurlmixin-get-domain
into 1a4de31 on develop.