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
98270704d3