If the existing email address for a user ends in `.invalid`, we should take the email address from an authentication payload, and replace the invalid address. This typically happens when we import users from a system without email addresses.
This commit also adds some extensibility so that plugin authenticators can define `always_update_user_email?`
A generic implementation of Auth::Authenticator which stores data in the
new UserAssociatedAccount model. This should help significantly reduce the duplicated
logic across different auth providers.
