🛂(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/impress/settings.py

@@ -384,6 +384,12 @@ class Base(Configuration):
     OIDC_STORE_ID_TOKEN = values.BooleanValue(
         default=True, environ_name="OIDC_STORE_ID_TOKEN", environ_prefix=None
     )
+    OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = values.BooleanValue(
+        default=True,
+        environ_name="OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION",
+        environ_prefix=None,
+    )
+
     ALLOW_LOGOUT_GET_METHOD = values.BooleanValue(
         default=True, environ_name="ALLOW_LOGOUT_GET_METHOD", environ_prefix=None
     )