Move eherkenning eenmanszaak flag to more natural SiteConfiguration

[swrichards]

This flag didn't really make sense in the OpenZaakConfig, it's site-wide: I've moved it together with the other login related flags in the general SiteConfiguration.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.19%. Comparing base (8c69698) to head (992f90f).

Files with missing lines	Patch %	Lines
...onfiguration_enable_eherkenning_for_eenmanszaak.py	82.35%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1597      +/-   ##
===========================================
- Coverage    94.19%   94.19%   -0.01%     
===========================================
  Files         1080     1082       +2     
  Lines        39699    39719      +20     
===========================================
+ Hits         37395    37412      +17     
- Misses        2304     2307       +3     

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