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
refs https://github.com/TryGhost/Team/issues/1111
refs https://github.com/TryGhost/Team/issues/1103
- moved customize modal from a link on the design screen to the main design screen
- changed modal design to be a full-screen overlay with side bar that emulates standard Admin design
- added toggled groups of settings in sidebar
- added `{{set-has}}` helper for use in conditionals matching when a Set contains an object
- added grouping of theme settings
- dropped unfinished advanced/change theme modals
refs https://github.com/TryGhost/Team/issues/1070
- split select form component into it's own component so it's cleaner when we get to additional setting types
- added change handler that updates the setting record's value when a new option is selected
- added `.isDirty` to the custom-theme-settings service so we can warn of unsaved changes and revert any changed values when needed
- added save of custom theme settings to the customize design modal's save routine
- added missing `notifications` service import to customize design controller
refs https://github.com/TryGhost/Team/issues/1045
- added tabs to the the customise design modal's sidebar for general settings and theme settings
- copied settings from the existing branding modal plus site description from general settings screen into the general settings tab
- theme settings tab left blank ready for static design
- `saveTask` put on the controller so that we can access it from the route, allowing us to pause modal closing when navigating away (implementation left for later)
