Some auth providers (e.g. Auth0 with default configuration) send the email address in the name field. In Discourse, the name field is made public, so this commit adds a safeguard to prevent emails being made public.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
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.
