Logging out of okta does not log you out of mediawiki #14

Pluggable Auth, when combined with okta, does a good job of logging you in. However, the session is kept internal to mediawiki, so if you log out of okta, that doesn't get pushed to mediawiki. In practice, this shouldn't be a big issue, but it does represent a security risk, and a usability error if multiple people are using one computer.