auth: Fix logout with Django >= 5 default authentication backend
Since Django 5.0, the logout view of the default authentication backend no longer accepts GET requests but only POST ones so adapt swh-web to take this change into account and enable proper logout operation when using the default authentication backend with Django >= 5 (it is used when running cypress tests or executing the webapp in docker for instance).
Also fix a couple of issues related to login/logout in templates and improve styling a bit.