[#2187] Implemented email-verification

[Bartvaderkin]

There is a setting in the SiteConfigurations (email fieldset) to enable it.

And the verified_email field is editable for testing.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.52605% with 14 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (develop@6d6d519). Click here to learn what that means.
Report is 12 commits behind head on develop.

Files	Patch %	Lines
src/open_inwoner/mail/views.py	70.37%	8 Missing ⚠️
src/open_inwoner/accounts/views/registration.py	93.75%	2 Missing ⚠️
src/open_inwoner/mail/verification.py	96.92%	2 Missing ⚠️
src/open_inwoner/mail/tests/test_verification.py	99.51%	1 Missing ⚠️
src/open_inwoner/utils/url.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #1126   +/-   ##
==========================================
  Coverage           ?   95.08%           
==========================================
  Files              ?      947           
  Lines              ?    33554           
  Branches           ?        0           
==========================================
  Hits               ?    31906           
  Misses             ?     1648           
  Partials           ?        0           

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[stevenbal]

Looks good overall, some minor comments