refs https://github.com/TryGhost/Team/issues/1130
- when opening/closing sections or interacting with the general design or custom theme settings we want to switch back to showing the preview so the sidebar acts more like typical navigation and so form changes show their effect
refs https://github.com/TryGhost/Team/issues/1111
We now have a situation where we have modals on modals and we've lost the straightforward built-in "Data down, actions up" communication methods that we'd have workaround across nested routes/controllers. The upshot of that is we didn't have a way to trigger a refresh of the preview when a new theme was activated.
- moved the task responsible for fetching preview html from the design modal onto the `theme-management` service and adjusted it to set a tracked `previewHtml` property rather than updating an iframe directly
- added a `<GhHtmlIframe>` component that renders a basic iframe element and updates it's contents each time the `@html` argument changes
- updated design modal preview to use the new iframe component
