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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/nephila/django-meta/pull/194/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/nephila/django-meta/pull/194/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila) | `97.82% <100.00%> (+0.19%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nephila#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

coverage: 98.913% (+0.008%) from 98.905% when pulling 6eb7b6429f9e87725e8f7b2d7fc32232b405aa73 on bugfix/issue-192-refactor-fullurlmixin-get-domain into 1a4de31e42bc236376a14044562c13fff7972adf on develop.