Updated magic link flow to allow changing member's email (#161)
Updated magic link flow to allow changing member's email (#161)

refs https://github.com/TryGhost/members.js/issues/30

- Updates `sendMagicLink` middleware to allow adding old email address to payload. Checks for if new email address already exists in db before creating magic link, throws error in case of duplicate email.
- Updates magic link parsing for data to check if the intention is to update email address and update member's email to new email address in case its allowed.
- Return session data from magic link using the new email address
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