[#1076] Fix/refactor case document upload form

[vaszig]

This has to do only with the upload form functionality, the refactoring after adding the ZaakType configs.
The submission part is covered by the next task (taiga #1062).

[codecov-commenter]

Codecov Report

Merging #445 (70354d4) into develop (0ea2953) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #445      +/-   ##
===========================================
+ Coverage    96.24%   96.28%   +0.03%     
===========================================
  Files          493      493              
  Lines        17380    17371       -9     
===========================================
- Hits         16728    16726       -2     
+ Misses         652      645       -7     

Impacted Files	Coverage Δ
src/open_inwoner/openzaak/api_models.py	99.17% <ø> (-0.06%)	⬇️
src/open_inwoner/accounts/forms.py	97.77% <100.00%> (+0.03%)	⬆️
src/open_inwoner/accounts/views/cases.py	97.40% <100.00%> (-0.02%)	⬇️
src/open_inwoner/openzaak/catalog.py	59.71% <100.00%> (+0.62%)	⬆️
src/open_inwoner/openzaak/managers.py	86.95% <100.00%> (+1.95%)	⬆️
src/open_inwoner/openzaak/models.py	100.00% <100.00%> (ø)
...rc/open_inwoner/openzaak/tests/test_case_detail.py	100.00% <100.00%> (ø)

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[Bartvaderkin]

👍 Neat!