An IFrame model type for external models

This is currently a work in progress making a pull request makes it possible to more easily see and comment on the changes.

There is some value to just having an iframe model that doesn't have properties connected. This would make it easy to get the 'interactive frame' around existing models like the AgentScript ones. That frame provides the 'share', 'about', concord logo, and fullscreen button. If we have non-lab interactives in the long run, it would probably be better to separate out this 'interactive frame' so a non-lab interactive could include it directly in its html instead of using an iframing approach to get it.