Leave control of is_superuser status with Django by default

[Andras1000]

In tokens.py the is_superuser status of a user will always be overwritten by this library.

user.is_superuser = bool(
    self.config.superuser_group
    and "groups" in claims
    and self.config.superuser_group in claims["groups"]
)

Make this code optional via settings so when an existing user is authenticated the already existing is_superuser status can be used.

I ran into this limitation as the groups scope is not returned in the flow unless one has the API Access Management feature in Okta which allows custom authorization server (see here), so could not use the SUPERUSER_GROUP setting.

Same applies to is_staff.

[samkuehn]

I am having the same issue. Here are my comments/suggestions on another issue.

#8 (comment)

[AzMoo]

Resolved in #28