Make possible to use plain content blocks

[krausandre]

As an integrator, I can configure my content blocks, so that my editors only see the content block fields.

Acceptance criteria

• [ ] Some content blocks can have the full range of the palettes
• [ ] Some content blocks have nothing but the Access-> enable field
• [ ] Possibility to configure over the yaml file

Acceptance tests

• [ ] Check if yaml file is handled
• [ ] Check if backend is rendering with or without default palettes
• [ ] Documentation is ready