## Is your feature request related to a problem?

Currently if you login with SAML and you don't have an account on the PostHog instance one of two things happen: a) either a new account is created **if the email matches the domain whitelist** or b) you get turned down at the door. Yet after some thought, the point of SAML is that you manage user provisioning in a separate centralized system, so we shouldn't have the extra domain check. 

## Describe the solution you'd like

If a user logins through SAML and their account doesn't exist, create it automatically, regardless of whether it matches the domain whitelist.


## Describe alternatives you've considered

Keeping as is.

## Additional context

There's a complicating factor which is that SAML login is instance-based yet user signup is organization-based, so there's the edge case of an instance having multiple orgs. Should the user then be signed up to all organizations?

#### *Thank you* for your feature request – we love each and every one!

posthog/api/signup.py
first_name: serializers.Field = serializers.CharField(max_length=128)
    email: serializers.Field = serializers.EmailField(
        validators=[
            validators.UniqueValidator(
                queryset=User.objects.all(), message="There is already an account with this email address."
            )
        ]
    )
    password: serializers.Field = serializers.CharField(allow_null=True)
    organization_name: serializers.Field = serializers.CharField(max_length=128, required=False, allow_blank=True)
    email_opt_in: serializers.Field = serializers.BooleanField(default=True)
    def validate_password(self, value):