refs https://github.com/TryGhost/Team/issues/1070
- adds a `customThemeSettings` service that handles overall setting loading and saving to avoid components having to know any of the intricacies of the model setup
- adds `custom-theme-setting-list` model so that we can save multiple setting records as embedded relations
- custom adapter ensures requests go to the `/custom_theme_settings` base route as a `PUT` request
- custom serializer drops the default `models: []` wrapper in the save request data so the format matches the `PUT /settings` endpoint, then converts the response to look like a `custom-theme-setting-list` response
refs https://github.com/TryGhost/Team/issues/1045
- update of the preview was being triggered by the general settings tab rendering (copy/paste from branding setup) meaning the iframe reloaded when switching between general/theme tabs
- moved the preview update task into the `customize` modal so that it can be called by the general/theme settings components as needed
- moved initial preview rendering to the iframe registration method
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)
refs https://github.com/TryGhost/Team/issues/559
refs https://github.com/TryGhost/Team/issues/1045
- removed need for `{{will-destroy}}` in the modal template by using `willTransition()` in the route that opened the modal instead
- used the property of new modals acting as promises to implement an unsaved changes confirmation modal
- added `confirmUnsavedChanges()` method that opens a "are you sure you want to leave?" modal that is treated as a promise. If the modal is closed by the "Leave" button the promise returns true in which case we rollback the unsaved changes. The modal is returned so that other behaviour can use this method and wait for the confirmation result
- added `willTransition()` route hook that calls `confirmUnsavedChanges()` when there are unsaved changes and will only continue to transition if the "Leave" button is pressed
- added `beforeClose()` hook to the customize modal when opening so that we can prevent the customize modal from closing when "Stay" is clicked in the confirmation modal
- updated `activate()` hook to store the modal instance so it can be closed later
- updated `deactivate()` hook to call `.close()` on the modal instance rather than using private data/methods on the modals service
refs https://github.com/TryGhost/Team/issues/1045
- added new `settings.design.customize` route
- used the route's `activate` and `deactivate` hooks to trigger and close the customize modal
- added `modals/design/customize` component that uses the existing branding modal design without it's contents as a base
- has `{{will-destroy}}` hook that transitions back to the settings screen so the route will update when the modal gets closed
