fix(oauth): Add `post_logout_redirect_uri` to OAuth logout redirect URI

[marvinruder]

This is a small addition to #631. Apologies for not including it earlier.

Without it, the OpenID Connect provider shows a “You are now logged out – Click here to go back to the application” confirmation page after logout. Adding the post_logout_redirect_uri parameter will cause the provider to perform the logout and immediately redirect back to Pingvin Share, reducing the number of user interactions and thereby streamlining the logout process.

This OpenID Connect feature is standardized here.

[stonith404]

Great, thanks :)