🛂(backend) match email if no existing user matches the sub

Some OIDC identity providers may provide a random value in the "sub"
field instead of an identifying ID. In this case, it may be a good
idea to fallback to matching the user on its email field.

src/backend/people/settings.py

@@ -413,6 +413,11 @@ class Base(Configuration):
     )
 
     OIDC_TIMEOUT = values.Value(None, environ_name="OIDC_TIMEOUT", environ_prefix=None)
+    OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = values.BooleanValue(
+        default=True,
+        environ_name="OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION",
+        environ_prefix=None,
+    )
 
     # MAILBOX-PROVISIONING API
     WEBMAIL_URL = values.Value(