Feature/6 meertalige content

[Floris272]

Changes

• Adds Parler
• Makes Product type translatable
• Adds translatable Content Element model
• Translate api response based on Accepted-Language header
• adds actions to product type & content element serializers to modify translations
• adds Wysimark editor

[Floris272]

The actual package @wysimark/standalone is not build correctly so i have used wysimark-standalone. Another option would be to build it within open-producten.

Currently npm run build calls collect (similar to how Open Inwoner does it) that copies the js file from node_modules to src/open_producten/static.

[Coperh]

I think it looks mostly fine.

• You could probably make the language API language simpler.
• It is possible to create translations for unsupported language.
• Content Label is not accessible via the API and is missing from the default admin index

[Coperh]

This will throw str after migrating since neither will be defined but I guess if no one has an existing DB, it should be fine?

[Coperh]

As in my product change page threw an exception because of this

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