[#1471] DigiD Machtigen

[SilviaAmAm]

Fixes part of #1471

In this PR the views/plugin/config/backend is kept separate from that of DigiD openID connect. We decided with Steven to do it like this initially because we are not sure if they will have different client ID / secret. If it turns out that more of the DigiD/DigiD Machtigen plugin can be merged then we can do a refactor.

TODO:
At the moment the same endpoints for keycloak are defined in many different places (the main OIDC config page, DigiD OIDC, eHerkenning OIDC page and DigiD machtigen page). Maybe it would be good to refactor the config models.
maykinmedia/mozilla-django-oidc-db#45

[codecov-commenter]

Codecov Report

Merging #1485 (281a714) into master (54e360e) will decrease coverage by 0.06%.
The diff coverage is 88.36%.

@@            Coverage Diff             @@
##           master    #1485      +/-   ##
==========================================
- Coverage   92.32%   92.26%   -0.07%     
==========================================
  Files         477      481       +4     
  Lines       14366    14616     +250     
  Branches     1313     1322       +9     
==========================================
+ Hits        13264    13485     +221     
- Misses        852      881      +29     
  Partials      250      250              

Impacted Files	Coverage Δ
...b/digid_eherkenning_oidc/tests/digid/test_admin.py	100.00% <ø> (ø)
src/openforms/urls.py	71.42% <ø> (ø)
...ication/contrib/digid_eherkenning_oidc/backends.py	43.58% <29.03%> (-56.42%)	⬇️
src/digid_eherkenning_oidc_generics/backends.py	42.85% <33.33%> (+0.54%)	⬆️
...ntication/contrib/digid_eherkenning_oidc/plugin.py	78.26% <65.00%> (-1.74%)	⬇️
src/digid_eherkenning_oidc_generics/models.py	97.72% <90.00%> (-2.28%)	⬇️
src/digid_eherkenning_oidc_generics/admin.py	100.00% <100.00%> (ø)
...rkenning_oidc_generics/digid_machtigen_settings.py	100.00% <100.00%> (ø)
src/digid_eherkenning_oidc_generics/forms.py	97.14% <100.00%> (+0.47%)	⬆️
src/digid_eherkenning_oidc_generics/mixins.py	100.00% <100.00%> (ø)
... and 6 more

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[sergei-maertens]

Ok with the decision to keep config separate and refactor later if we observe a lot of code duplication.