refs https://github.com/TryGhost/Team/issues/1441
- switched "leave settings" confirmation modal on members email settings screen over to modern modal pattern
- removed unused `showEmailDesignSettings` property and `closeEmailDesignSettings()` action on `<Settings::MembersEmailLabs>` component
- the used property and action live on the controller, looks like it was a copy/paste hangover when functionality was moved to a component
- added newsletter model
- includes design-related attributes which are not yet supported by the API but are due to be added
- includes `default` attribute but there is no setting for it, due to be removed from the API but it's needed for save not to error for now
- set up basic mirage model and endpoints
- added validation for main settings to match API validation
- added `EditNewsletter` modal
- separate tabs for general newsletter settings and design-related settings
- used for both creating and editing newsletters
- added `/settings/members-email/newsletters/new` and `/settings/members-email/newsletters/:id` routes
- both display the `EditNewsletter` modal on top of the members-email settings screen with the appropriate newsletter model
- updated `<Settings::MembersEmailLabs::NewsletterManagement>` component to work with real newsletter model instances and the new add/edit routes
- removed now-unused `newsletter` service that was providing mocked data for earlier design iteration
6e0be9e175