Discourse doesn't re-verify an address changed by SSO

[DeanMarkTaylor (Dean Taylor)]

Change the email address to some other existing user, sign on using SSO, it will update the external_id of that user.

Change the email address back, now the external_email of that user is updated as well.

This goes against the requirements set out in the SSO topic:

Official Single-Sign-On for Discourse (sso)

external_id is any string unique to the user that will never change, even if their email, name, etc change. The suggested value is your database’s ‘id’ row number.