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.