[#811] Case notification base code

[Bartvaderkin]

Let's review this part before I gets out of hand:

This is everything to accept notifications from Openzaak, including filtering the case/status/role/users, up to actually sending the message.

So these are basically two parts: the generic ZGW notification webhook and the first of the OIP specific part, but the PR touches a bunch of unrelated things because I updated the TimelineLog system-action wrapper.

I have a branch on top of this that adds the ZaakTypeConfig (to also handle eSuite's' informeren) and sending the actual email.

(not a lot of this would run locally so if the tests look good it's good)

[codecov-commenter]

Codecov Report

Merging #435 (a8d3233) into develop (74d1813) will increase coverage by 0.18%.
The diff coverage is 97.79%.

@@             Coverage Diff             @@
##           develop     #435      +/-   ##
===========================================
+ Coverage    96.32%   96.50%   +0.18%     
===========================================
  Files          472      482      +10     
  Lines        15747    16330     +583     
===========================================
+ Hits         15168    15760     +592     
+ Misses         579      570       -9     

Impacted Files	Coverage Δ
src/open_inwoner/urls.py	87.50% <ø> (ø)
src/open_inwoner/openzaak/cases.py	76.00% <50.00%> (+5.33%)	⬆️
src/open_inwoner/openzaak/notifications.py	92.70% <92.70%> (ø)
src/open_inwoner/openzaak/auth.py	94.11% <94.11%> (ø)
src/open_inwoner/utils/tests/helpers.py	95.00% <95.00%> (ø)
src/open_inwoner/openzaak/api/views.py	97.95% <97.95%> (ø)
...nwoner/openzaak/tests/test_notification_handler.py	99.37% <99.37%> (ø)
src/open_inwoner/accounts/query.py	100.00% <100.00%> (ø)
src/open_inwoner/accounts/tests/test_logging.py	100.00% <100.00%> (ø)
...c/open_inwoner/accounts/tests/test_user_manager.py	100.00% <100.00%> (ø)
... and 26 more

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[alextreme]

Okay, lets give this a try on the testenvironment due to the dependancy on OpenZaak/OpenNotificaties