refactor(DataObjectAsPage): Make this class more flexible

Removes Content field
Removes call to parent::getCMSFields so we don't get unwanted scaffolded fields
Extracts field creation from getCMSFields into separate protected methods that derived classes can use

BREAKING CHANGES:
Content field has been removed - derived classes will have to add this back themselves if it is needed.
Scaffolded fields are no longer included in getCMSFields - derived classes can simply use an instance of FormScaffolder to achieve the same result.