Corrected calculation of UserSocialAuthMixin.expiration_datetime

Corrected calculation of "now timestamp" in utc.

`datetime.now()` returns a datetime in local time zone so by doing now.replace(tzinfo=utc) you are adding/removing time (depending on your configured local timezone)
I changed it to use `now_timestamp` because it is calculated with `time.mktime` and it takes into account time zones.

If you were if the side of the word where "UTC offset > than provider session time" the function `UserSocialAuthMixin.expiration_datetime` returned a negative time that forced the expiration of the django session.