Rewrite CustomUser's save() method to exclude creating a tenant for AnynymousUser

[igorsimb]

django-guardian creates AnonymousUser by default, it does not need a Tenant since it's not used by users Some tests will fail since they rely on this user's tenant, which is not desirable.

• [ ] Fix save() method
• [ ] Fix tests

CustomUser model could look like this

class CustomUser(AbstractUser):
    tenant = models.ForeignKey(
        Tenant, on_delete=models.CASCADE, null=True, blank=True, related_name="users", verbose_name="Организация"
    )

    class Meta:
        indexes = [
            models.Index(fields=["tenant"]),
        ]

    def save(self, *args, **kwargs):  # type: ignore
        if not self.tenant and self.username != "AnonymousUser":
            self.tenant = Tenant.objects.create(name=self.email)
        # django-guardian creates AnonymousUser by default, it does not need a Tenant since it's not used by users
        elif self.username == "AnonymousUser":
            self.tenant = None
        super().save(*args, **kwargs)