Editor remains editable in readonly mode

[thclark]

Bug report

What's happening

The json is still editable when widgets are rendered as read-only in the admin (eg for fields where editable=false).

Cause

In the props passed to the component, props can now be customised, so a readOnly prop is acceptable. But, its value isn't determined; it must be set explicitly on the widget or in settings.

The admin governs whether a widget should be readonly or not. So, the readOnly prop (see editor customizable properties) should determined, rather than treated as a customizable option.