refs https://github.com/TryGhost/Team/issues/1310
- we had a `<GhBrowserPreview>` component but it wasn't being used everywhere because it's behaviour didn't match the mobile/desktop switch with no iframe re-render that we needed for some screens
- updated component to match all required behaviour
- include a switch between desktop and mobile views
- default fallbacks for `@title` and `@icon` arguments
- switched design settings themes over to using the component instead of repeating the same set of markup
