use default session cookie domain

[dakaza98]

Description of the Change

Currently, if you sign in to one website, you are signed in on all the other subdomains. For example if you sign in to utn.se you are automatically signed in to apply.utn.se, balen.utn.se etc. This is because all subdomains share the same session and it comes from the setting SESSION_COOKIE_DOMAIN in settings/production.py which is set to .utn.se i.e. all domains that end with utn.se. However, you can't sign in on utnarm.se, as mentioned in #449, since it is a different domain from utn.se and this setting is the reason why. This PR solves that

The solution is to remove this setting and go with the default and traditional behavior which is that each domain has their own separate session, as described in the django documentation. This means that if you sign in to utn.se, you will no longer be automatically signed in to apply.utn.se or any of the other domains. Instead you will have to sign in to them individually. But this will make it possible to sign in to utnarm.se and any other domains in the future.

Applicable Issues

Closes #449