Create new API endpoint for email optin

Creating a new Email Opt-In Endpoint in the profile API for ECOM-525, sub task ECOM-685. This hooks into the new User Org Tag model. 

@dianakhuang @rlucioni
Is `now()` in UTC?  (It's not super important, because this is such a coarse-grained check, but elsewhere in the platform we use `now(pytz.UTC)`)