Fetch published content

Move content controller to CMS and add active_model_serializers
Wdyt about we combine this two ternaries in these two methods above in one line statement:

```ruby
   def content
      return object.content if scope == 'preview' || object.blockable.state == 'published'

      published_content.fetch(:content, '')
   end
```

Of combine this two in if/else statement.