Fix logout

[huwshimi]

Correctly remove the login cookie. Fixes: #3946.

[webteam-app]

Starting demo at: https://juju-gui-juju-pr-3954.run.demo.haus/

[huwshimi]

QA:

• visit /login and log in
• visit /apache2
• you should see the charm in the GUI
• click logout in the menu (or visit /logout)
• you should get logged out and redirected to jaas.ai
• visit /apache2 (at your gui domain/ip)
• it should redirect to jaas.ai/apache2