refs https://github.com/TryGhost/Team/issues/1513
- `sender_name` is now nullable with a fallback to the site title
- updated new-newsletter route
- removed default setting of site title in `senderName` of the new newsletter instance
- removed extra checks when showing unsaved changes modal as we no longer need to compare the `senderName` attribute against the site title
- updated newsletter preview so the sender name falls back to the site title
- updated sender name input placeholder to show the site title
- removed not-empty validation
- fixed the switch to "multiple newsletter" state in the background of the new-newsletter modal
- problem was `displayingDefault` getter was looking at all active newsletters rather than just the filtered ones so it was counting the new-but-unsaved newsletter even though it wasn't displayed in the list
- fixes layout glitch when the new-newsletter modal animates out after cancelling creation
refs https://github.com/TryGhost/Team/issues/1492
Allows site owners to manage member's newsletter preference directly in Admin.
- also adds `visibility` property for newsletter model
- updates members test model to include `visibility`
refs https://github.com/TryGhost/Team/issues/1500
- `senderEmail` is now nullable with a fallback to `noreply@{site domain}`
- `senderName` is not nullable and has no fallback
- updated preview and input placeholder to match real-world behaviour
- inserted site title as the default value when creating a new newsletter to avoid friction with the sender name needing to be filled in when saving
- switched `senderReplyTo` input field to a select with "newsletter" and "support" options
- added basic `<Inputs::Select>` component as `<OneWaySelect>` had re-rendering issues causing loss of selected value when the label of the newsletter email changed to reflect a custom newsletter email value
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
