refs https://github.com/TryGhost/Ghost/security/advisories/GHSA-65p7-pjj8-ggmr
The email address change flow was built on top of the unauthenticated
signin/signup flow. This meant that ownership of the email being changed
wasn't verified and allowed a malicious actore to change the email
address of arbitrary accounts to an email address which they controlled.
We remove the ability to change email addresses from the signin/signup
flow and instead create a dedicated, authenticated flow for changing
email address.
