Login is broken in Discourse 2.8

`UserNameSuggester.suggest` was changed in discourse/discourse@19d95c64af9d38c7e34173f96745ae040ef0190e; that broke a [call](https://github.com/paviliondev/discourse-wikimedia-auth/blob/fcd7003c5ecad9438c5918089b1e4e22524b0586/plugin.rb#L15) in the plugin.

Downstream task is [T296603](https://phabricator.wikimedia.org/T296603).