Change CommonMark attribute to show rendered output when attribute is readonly

[stevehansen]

Description

https://github.com/2sky/Vidyano/blob/master/src/WebComponents/Attributes/PersistentObjectAttributeCommonMark/persistent-object-attribute-common-mark.html#L7

At the moment the attribute will only be rendered when not editing, but if the attribute can't be changed it will still show the textarea as readonly text.