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
