Adds new helper `content_settings_value`

This helper returns a value either from `content.settings` or from `options` hash.

What this brings?

You can now set essence view partial options in the `elements.yml` file:

```
- name: image
  contents:
  - name: image
    type: EssencePicture
    settings:
      image_size: '100x80'
```

This also adds a lots of specs for essence views and editor partials.