Unique constraint on nonce missing

[candlejack297]

The docs say,
SOCIAL_AUTH_NONCE_SERVER_URL_LENGTH = <int> Nonce model has a unique constraint over ('server_url', 'timestamp', 'salt'), salt has a max length of 40, so server_url length must be tweaked using this setting.
but I don't actually see this being enforced anywhere nor any reference to it in the migration or model.

[omab]

Closed by #491