refs https://github.com/TryGhost/Team/issues/1130
- added `ui.contextualNavMenu` property that when set will switch the main nav menu between components, if it's not set then it will show the default "main" menu component
- added `design` menu sub-component of `gh-nav-menu` ready for use via `contextualNavMenu`
- moved sidebar contents from the design modal into this component
- updated design route to set/reset `ui.contextualNavMenu` to `design` on entering/leaving the route
- all other design routes are sub-routes so this works across all other design screens
- moved base design modal preview into the `settings/design/index` controller/template
- using index means that it's the default screen for `/settings/design` but will be automatically replaced by any other `design.x` routes
- moved `design/advanced` modal content into the `settings.design.advanced` controller/template and removed the modal handling from the route
