refs https://github.com/TryGhost/Ghost/issues/11414
Confirms if the fromAddress for sending member emails is valid and accessible using magic link flow, allowing owners to update full from address including domain change.
- Extends member service to handle magic link generation and validation for email update
- Updates existing setting endpoint to not directly update from address
- Adds new endpoint to send magic link to new address
- Adds new endpoint for validating the magic link when clicked and update the new email for from address
- Adds new email template for from address update email
