[#11] add eigenschappen

[Floris272]

Fixes #11

Changes

• Adds producttypen.Eigenschap & producten.ProductEigenschap models
• Eigenschappen can be created/updated within the producttype serializer
• ProductEigenschap can be created/updated within the product serializer using naam/waarde

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.38%. Comparing base (f00b267) to head (21912b0).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
...n_producten/producten/models/product_eigenschap.py	84.00%	3 Missing and 1 partial ⚠️
...open_producten/producten/serializers/validators.py	81.81%	2 Missing ⚠️
...c/open_producten/producttypen/models/eigenschap.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   84.92%   86.38%   +1.45%     
==========================================
  Files         106      118      +12     
  Lines        2269     2526     +257     
  Branches      149      160      +11     
==========================================
+ Hits         1927     2182     +255     
+ Misses        319      318       -1     
- Partials       23       26       +3     

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